HELP: AutoCAD slows down

We just got new computers at work. Dell P4 2.8 512 ram 40 gig HD.
Wer'e running Windows 2kPro and AutoCAD 2002. After working in AutoCAD
for about 20 to 30 minutes all the commands slow way down, another
words if I pick the move command there's about a 5 or 6 second delay
from the time I pick it to the time it initializes. If I turn AutoCAD
off and then on again it works fine again for about twenty or thirty
minutes and then slows down again. I checked the virtual memory
settings and they seem right (1.5 times the abount of ram min.). I
tried turning off all other programs but AutoCAD and the same problem
occurs. Computer has 512 meg of ram which should be plenty. Anyone
have any idea what is wrong??