PT freezing under 5.1
 

I was using 5.0.1 on a G4/400 for about four months without any problems and recently upgraded to 5.1 (I also added 512 MB of pc133 RAM, but have tried this test without it). Since the u/g, PT freezes after twenty minutes work or even leaving it looping a portion of a track. I get the taping fingers and no response from the keyboard (Apple/Option/Esc) or mouse. I have to restart the G4 and start again.

A call to Digidesign suggested that PT is waiting for computer hardware. However I can run a 500MB loop of video in 'Strata Video Shop' (ships with Fuse card) for hours without any freezes.

I have tried running minimal extensions, core + PT, still freezes.
Any suggestions as to what else to check/test would be appreciated. Is anyone else experiencing this ?
Many thanks in advance, Barry