Simon's method is best. I have done this method dozens of times.

Its time consuming but it's the best and only way to resolve issues with capture cards. (Also the same method for the Phoenix Cards with the exception of the driver file.)

I have still seen this happen regardless of any troubleshooting. I am dealing with this very issue in fact in a nightclub. Fortunately I am the only one who notices but it still drives me nuts.

My speculation is electrical noise and subtle phasing problems which cause the digitizer to perceive a drop. I can't confirm this but nothing else makes much sense.