Thanks for your reply. Sorry I haven't got back sooner - my connection has been down
Anyway, glad to say the problem is now fixed. Not sure what was causing it, but by resetting to default settings using the quake menu I made it go away.
This seems to suggest that the problem was something like having mouse smoothing enabled, but I'm sure I tried all that. Never mind - it works now anyway
Thanks again for your suggestion.
reetep.