Loosing TCP/IP on Workstations

I have an intermittent problem, which has occurred on 2 laptops and one desktop PC. It has happened under both 95/98. They all have a customised version of IE5.01.
The users log in and all seems fine. They get IP and they use the INET. When closing IE5 and going back in, they get the page can not be found error. While checking further, if I use winipcfg I’d get a fatal error: IP not found. When pinging the gateway, no response. The only recovery is to reboot, but it is happeningquite frequently. One of the guys is our web admin, so you can imagine the dilemma. For further info, we do have a proxy server, but I think this is irrelevant. I’ve tried searching the Microsoft site but found nothing. Anyone have any ideas?