Random Crashing of NT 4.0 Server

We have an NT Server which crashes randomly.

The hardware, which is a dual Pentium system, has tested OK and all cables & network cards are also OK.

Most PC’s on the Network (approx 40) are of a reasonable spec and the room the Server is housed in is well ventilated.

The system seems to run fine for a few weeks,then gradually the Network get slower, before eventually just crashing.

Does anybody have any ideas?

Many thanks.