basically there are three or four pc's on our network of thirty pc's where intermittently the machine locks up and when you try and move the mouse or press any key it makes a beep beep beep noise as though some kind of buffer is full these pc's are on an nt4 network (cat 5)these pc's are using windows 98 i have tried changing network cards but to no avail all the machines developed the problem at roughly the same time, any ideas ?