Timing issue

Hi group:

I'm using psychtoolbox for simple fMRI experiments which mostly call
on simple Screen commands. Recently, I received the following error:

PTB-ERROR: Screen('Flip'); timestamping computed an *impossible value*
1633.600234, which would indicate that stimulus onset happened
*before* it was actually requested! Something is broken in your
systems [sic] timstamping. High-precision timestamping disabled...


There is some more to it, but that's the general idea.

Since then, none of my experiments will function.

Two questions:

First, what can I do to fix this? Obviously, I have deleted old files
and have reinstalled matlab and psychtoolbox.

More importantly, how can I prevent this from happening again?