Loss of network connections

when xp is running normaly i have network connections both to other pcs and the internet but after anything from 2mins to 30mins the pc drops the connection, on the send(tx)the tx drops to 0 but the Rx carries on with the packet reports,(network controler)(2little monitors by the clock)
if on the other hand i start the pc in safe mode with network support it will run all the time..
But when in normal running,with all the software switched off as if it were in safe mode it will run for about 1 or 2hrs before it drops connections……..

any idears cos this is driving me mad…