Thanks for your input borate
A little more info gleaned in the meantime...
Have been playing some more, and although I got things moving again (not sure what specifically I had done) I can fairly confidently state the problem in all this is x264enc7.exe as the jerky playback mentioned above is due almost entirely to the active presence of this item. I cannot see why it is running when it is. It often runs ad-infinitum and well beyond any visible signs of doing something useful, and in fact, just killing the process has no visibly adverse consequences, and does restore smooth scrolling instantly.
When the process is not present scrolling on the timeline is as sweet as ever.
Judging by the many hits typing that process name into Google produces, there's quite a lot wrong with that item and I conclude its not just VP users.
1) whats does x264enc7.exe do?
2) when does it do it?
3) if deemed unnecessary can it be permanently removed without consequences?
4) does deleting VideoPadCache structure remove all cached data VP generates?
5) how does the greenline cached indicator appear instantly for a clip VP has seen earlier, even though the cache has been cleared? My assumption always having been that caching the timeline produces smooth scrolling and the data for which was held in the cache folders, to save re-chaching next time the project is loaded.
It would seem there are times when x264enc7.exe does appear and fairly promptly disappears, but there are times when it fails to cease and just sits there consuming loads of processor power doing nothing.
I will check those points you raised above Borate just in case they help, and again, thanks for yours.
Alan