Network Shuts Down



I have a problem with my 1.8 Ghz PM G5 (OS X 10.4.2).

When the computer has been on for about 2 hours and 32 minutes (+- a few minutes), the network shuts down and I have to restart the computer in order to get online again. I have a router (I have tried to restart the router but it doesn't help) and I have unchecked all the options in the Energy Saver preference pane. Also when the network is down, the computer cannot go to sleep because this makes it freeze. Also when the problem is occuring, it takes about 2 minutes for the computer to restart or shut down.

I have searched everywhere but still haven't found a solution. If you could help me with this I would appreciate it very much.

Thank you!