I presume you're well aware of the Live Update switch at the top of the timeline. And that you have a fully-updated NVidia driver for your card.
Yes I am, Dave, thanks, and no it's not that.
Preview mode is set to adaptive resolution and the timeline updates as I scrub, but intermittently the content across all layers will cut out visually for a second or so - showing only the coloured inverted "V" image place markers. The image returns the moment I stop scrubbing.
I would expect the image to pixellate to a lower resolution whilst scrubbing - if it can't update fast enough - but it doesn't appear to do so.
Also, this doesn't explain the behaviour when using the pen tool (oultined above)....
As I said, this was not a Quadro FX3800 issue.
As an aside, In Adobe Forums, the text entry panel on my PC has started to display in Times Roman all of a sudden, but changes to the correct display font when posted. It's fine on my Laptop and iPad.
Am I having another senior moment?
I can only put this down to the vagaries of the GTX680.
When I preview out to a third monitor via a BM intensity, I have continuity of output on the third monitor whilst scrubbing.
Conclusion: I'm not sure of the technicalities of memory flushing, but superficially, it appears that the 1Gb FX3800 is more capable than the 4Gb GTX 680.