Router "ignores" iMac network traffic after a short while...

someone here might be able to help. Here are my symptoms:
My home network consists of Comcast broadband connected to an SMC 802.11N broadband router, which then feeds into several switches and several other wireless access points in my house to support the various dead spots (3 levels, big enough that I put in more AP's over the years.)
I work in IT for a living. All APs are on separate channels, and I use iStumbler to make sure I am not being stepped on by my neighbors. I also use a radio frequency analyzer to watch for spurious emissions from cordless phones, etc. In any event, I do not believe this is a WIRELESS issue as you will see in a moment.
What happens is that I can get my new iMac onto my network and online with no problem...I can do this by connecting to the main SMC router wirelessly, or by connecting wirelessly to any of the other APs in the house. I get great connection speeds (20+MBit as reported by Speedtest.net) and can surf just fine...for a while.
What happens is that my SMC router eventually starts to "ignore" my iMac. What I mean is that I can turn on a network sniffer and watch the communications between the iMac and the router working just fine...then all of a sudden the SMC router starts to ignore ARP requests from the iMac. Since this main router is my gateway on the network, this means that the iMac can longer get out to the Internet. BUT, I can still reach other local resources on my local network via the other AP's just fine. For example, I have SMB shares on other machines on the network and if the iMac has already grabbed the MAC address for these local machines, when the router begins to ignore my iMac it can still talk to local machines just fine. The router just 100% ignores the iMac now (pings, logins to the web admin console, ARP requests, etc).
I can recover by dropping the wireless connection from my iMac to the non-router AP and RECONNECTING to the router's built in access point. This is obviously not a sustainable work-around however. I can also restart the router and recover.
I thought that perhaps the register on my router was being over-run with traffic from my iMac and read that something like DropBox may do this. I AM running DropBox on all my machines so removed it from my iMac...no luck. My SMC router starts to ignore my iMac after 10 minutes or so of surfing even with no DropBox app running on my iMac.
There are no errors in the SMC's logs that would indicate a problem, and none of my Windows/Linux machines are having any issues with this SMC router.
Having said all that...I anticipate that the first response to this post might be, "it's your router's issue...replace that and all is well." But I am struggling with the fact that 7+ other machines on my home network (ranging from variations of Linux to Windows 7 to Windows HomeServer) all interact with this SMC router all day long...it only starts ignoring my shiny new iMac after 10-15 minutes.
I also have not tried to reproduce this by connecting directly to the SMC router (main gateway) to see if it happens. Again, I am going iMac -> Wireless Access Point -> Switch -> SMC router so not sure why this would matter or not when trying to determine why the SMC router and iMac refuse to communicate after a short while (and I need to use my new iMac wirelessly anyway).
any suggestions?
Thanks!

we are having the exact same symptoms where our apple products are connected, but can't access the internet. we have the same router from Comcast. it is an SMCWBR14S-N4.
we have an iphone4 and ipad2. used to think it was something weird about the iphone, which we've had for months, and we've had this problem for months. just got the ipad last week, and it immediately started showing the same symptoms.
like you, we have other computers, and they don't have these issues. we also have a panasonic viera flat screen with a 802.11n dongle. none of these machines have an issue.
any luck getting your problem solved?

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