we had similar experiences with an architect in Berkeley, and we couldn't really pinpoint it, at one point trying even the delete and reinstall.
It seemed that once you quit archicad and restart the system it worked again for a while, but we didn't have enough time to figure out which application (classic?) causes the trouble.
Throwing away the preferences Gergely so ably pointed to didn't help us, but it might be microclimate
So it says a mistery, but we need to get the jobs out the door...