Slowdown over time with Remote desktop

I use remote desktop between two XP PC's.

One PC is left running and logged in as it drives a piece of
instrumentation. The other is in my office and is regularly logged in and
out. When I initially connect to instrument PC from the office based PC it
works fine. It's over a LAN so there's no problem with connection speed.

If, however, I try the connection after a day or so, the screen freezes and
I can only see what's changed by minimising and restoring the remote screen.
If I then restart the remote pc everything's fine again for another day or so.

Any ideas what might be happening?

I should also add that if I actually walk up to the instrument PC and use it
that way there are no problems.

Mike