Video Memory Degradation - intermittent black screen

performance IndicatMy screen has been intermittently going to black for 1/2
second or so - usually a couple of times in succession every 15 minutes or so
- the frequency varies.
I have a Dell Dimension 9200 with a256MB PCle x16 ATI Radeon x 1300 Pro card
and 1280 x 1024 screen resolution. I contacted Dell, ran the monitor's self
testing, increased all the performance settings on the graphics card and
downloaded two semingly relevant Vista updates (KB938194 and KB938979) but
the monitor continues to drop out and now I have received a 'Performance
Information and Tools' pop-up saying 'Adjust visual settings to improve
performance' and directing me to 28 Warnings in the Event Log.
All the warnings are Event ID 500: 'The Desktop Window Manager is
experiencing heavy resource contention Scenario: the Desktop Window Manager
responsiveness has degraded.' I'm not sure how to respond.
The monitor screen continues to intermittently go to black - any suggestions?