I've seen this a couple of times in the news groups but with no actual answer.
What is the resource usage like on the system? How does the resource usage compare when the tip isn't open?
Some have suggested upgrading to the latest version of the Wacom drivers but I don't know if that has resolved the problem for anyone. http://www.wacom.com/tabletpc/driver.cfm
Worth a shot though.
If it's only just started happening then you can look at all the obvious stuff - has anything been installed recently etc. that may affect it. How about trying system restore back to when it was working (if you still have a valid restore point).