Internet speeds keep slowing down to a crawl

Question

I'm having a problem that has only developed in the past week which was during a time when I haven't changed any configurations in that period of time since my son was born a week ago yesterday and I obviously had more important things to do than mess with my network configurations. However, about 2 weeks prior to his birth, in the process of configuring a baby monitoring system with icam source, 3 weeks or so ago, I read that I might be able to improve my internet performance, I ran googles namebench a couple of times and added and re-ordered the DNS servers that had been listed prior to any problems developing based on its' reports. My internet speeds improved as a result of changing those DNS settings and stayed good until the past week. But of course, we now need to use the computer as part of our baby monitoring system and it's not working worth a **** and I'm getting frustrated since I can't put my finger on the problem and I don't really have time to spin around in circles right now. I'm sure it's probably something simple and I can't put my finger on it though. Sleep deprivation is new to me.......
I've kept running log of notes so this is a little deshoveled but I figured the more information I provide the better the chance someone might help me get this solved quickly.
In the last week, since my sons birth, my internet speeds would be okay after restarts and resetting everything, 2.5 Mbps or so but after a 10-15 minutes it starts to slow down to .2-.3 Mbps. I went thru all my network settings, restarted everything, repaired permissions, I've changed ethernet cables reset PRAM, firewall is running, stealth mode enabled, allow signed software incoming connections and my various other programs such as netsessionmac4f3c6b, skype, java, itunes, etc.
All sharing services off and pretty basic login items at startup including growl.
I had AT&T come out and check my DSL line and everything checked out on their end up to and out of the DSL modem based in their equipment tests.
The problem seems to start when I connect my imac (a fairly loaded 27" imac I got in Dec.) First off, as soon as the imac is connected to the modem via ethernet, the old ethernet modem flashes and continues to flash, even right after startup. So the tech thought something was running in the background and I can't figure out what. I looked through login items but don't see anything that would be a problem and am afraid to eliminate everything.
So I created a guest account to eliminate any login items (I think) and in I got 5.69 Mbps right off the bat. But after a minute or two, without changing anything my download speed slows down and I get for example .77 Mbps. The same thing happens with my administrator account on the imac. Fast speeds at startup and more importantly if I change the configuration by switching between ethernet and airport, I get fast speeds right after changing the configuration but after a minute or two I am well below 1.0 Mbps. I've got several network configurations with automatic, airport and ethernet set up in different ways including a clean ethernet connection with the original DNS settings.
I haven't set up an airport only account but that really isn't the way I want to hook up the computer that sits right next to the modem and runs my network. Restarting the old modem and direct connecting the computer to the modem results in the same thing happening.
I connected my ethernet from the (old?) modem directly to my ancient powerbook G4 and I got +5.6 Mbps download but it also slowed down to 300 kbps after 5 minutes or so the first time (old modem?)
Now to explain why I have "old?" listed above. I was then able to get a new DSL modem from AT&T, hook it up just fine, get enormously faster internet service in the 5,000 kbps only to have it slow back down to around 300 kpbs after connected for several minutes.
directly connected to imac-bypassing router
3,500 kbps
after 15 minutes-250 kbps
connected to imac thru router/time capsule with airport extreme
2500 kbps
after 15 minutes=166 kbps
directly connected to an older powerbook G4 with new modem
IP address of 192.168.1.64
5.65 Mbps on speedtest
after 15 minutes=5.67 Mbps
after 30 minutes= 5.59 Mbps
1 hour-5.66 Mbps
Very solid, stable readings for what I am used to. This would seem to isolate the problem to the imac.
directly connected to imac with new modem
IP address of 192.168.1.64
5.8 Mbps on speedtest
after 30 minutes= .60 Mbps-ping takes 250 ms
1 hour= .46 Mbps-ping takes 250 ms
Now a problem that only started last night after I hooked up the new modem and started up my old Time capsule/airport configuration, I got a message that said something to effect of "another device on this network is using computer IP address 192.168.1.64. So I tried manually changing last digit of the IP address on the computer to one number higher and it kept saying the start and end range were incorrect so I shut the airport station down since when it was connected, it would not allow the computer to connect thru the router via ethernet. I did a hard reset on the airport and I got it back up and running just fine but the internet speeds on the imacs' ethernet connections are so slow it's practically worthless. I also checked to see what was connected to my airport on any given occasion, 2 iphones, 2 pads, another imac running upstairs and a newer powerbook running icam source that is supposed to be the baby monitor. Except for the laptop everything can use "802.11n" but the powerbook only connects via 802.11g so I have set the airport to that limiting setting.
Any suggestions? Is there a way for me to log what is happening so I can report in more detail?

Answer 1

First thing is to check if the wireless is on a sensible channel: HomeHub doesn't always make sensible choices.  Install inSSIDer on your laptop and see what other devices there are around and if there is any clash.  Also, try moving the HomeHub around a little, even quite small differences can make a big difference to wireless signals.
You could run an ethernet cable from downstairs to upstairs.  Easy to do, but can be a bit unsightly.  If most of your devices are upstairs, you could use this cable to connect the modem to the HomeHub.  If you have devices both up and down, you may want to keep the HomeHub downstairs and add a switch upstairs.
HomePlug may be suitable for you.  If you search Amazon for HomePlug you will see a couple of kits for just over £30.  Don't go for an 85Mbps one.  A 200Mbps such as the TP-link or D-Link should be much better than wireless, but not as good as a cable.
Depending excatly how many devices you want to connect and where, you can pay more and have a builtin wireless at one end: http://www.amazon.co.uk/Devolo-1408-Wireless-N-HomePlug-Adapter/dp/B00472MMCQ/ref=sr_1_15?ie=UTF8&qi...  Or it may be best for you to use the basic devices and add an old wireless router in switch mode.