Locked up and cannot restart

I have a Win98 machine acting as a server for 4 Win95 machines. The 95
machines have mapped drives to a shared data folder and a shared application on the 98 machine. The problem happens between the 98 machine and just one 95 machine.Sometimes theywill both lock up and the only way to restart is to unplug the machines. No Ctr-Alt-Delete and the power buttons will not work. Any ideas what could be causing this?