Extreme internet lag and inability to connect to other macs in network..

Question

Hi,
My network is as follows:
Linksys AM300 half-bridge mode to Apple Airport Extreme (latest edition) routing to:
-1 Mac Pro (2008) via ethernet
-1 iMac (2008) via wireless-n
-1 Mac Mini (2003) via wireless
-1 windows xp laptop via wireless
-1 MacBook Air via wireless-n
The distance from Mac Pro to the Airport is relatively far but the signal is good and internet exhibits no lag whatsoever.
The MacBook Air is in the same room as the Mac Pro. Since the distance to the Airport only gives a mediocre internet connection I use my Mac Pro's internet sharing to enable its airport as an access point with WEP encryption then connect my MBA to the Mac Pro.
Until a couple of months ago everything was fine but now I suddenly cannot connect to the screensharing or filesharing services of the Mac Pro and the internet connection has become ridiculously slow. I experience a roughly 20 second lag upon EVERY request to any site (google.com for example) and using the terminal i pinged google which yielded an average ~300ms response.
My browser is a Firefox 3.0.6 btw, this is also the browser on the iMac and Mac Pro.
While the lag is annoying even more so is the fact that I cannot use the MBA to 'remote control' the screen of my Mac Pro. The Mac Pro has not had its settings or software changed between working and not responding.
Connecting to the Mac Pro from the MBA via the regular Finder -> Network dialog shows the Mac Pro but then responds with a basic "Connection Failed" message. The same happens when trying to connect to the Mac Pro from the iMac or Mac Mini. I haven't tried the windows laptop so far.
All other computers in the network (other than the MBA) have 'normal' fast internet speeds and no lag.
The lag is also present when sitting next to the Airport Extreme circumventing the internet sharing of the Mac Pro.
I have no clue what is going on and suggestion and help would be much appreciated.
Thank you very much for reading.
M.

Answer 1

The first thing to do in this sort of situation is to use Network Utility to ping to/from the server and clients to see if they can reach each other. Network Utility will also tell you what IP address the Mac you are running it on has. If you are using a 192.168.x.x style address range as per your message then they should all be in the same block e.g. 192.168.1.x where x is unique per computer.
The next thing may be to look at your DNS settings. You don't say what services you plan to use on the Mac OS X Server, but if your going to use Network Login Accounts and Network Home Directories then having your Mac OS X Server as the (working) DNS server will be very important. This would involve normally telling the DHCP server to advertise the Mac OS X Server as being your internal DNS Server.