I have this problem in PS CS4 and 5. I am using Intuos4 with newest drivers, older drivers do not make a difference. Nothing like this happens in any other application. The same thing happens when I map pen pressure to opacity rather than size as pictured, i.e. there is a tail of identical opacity at the end. I have a reasonably good PC; I don't think that's an issue (3Ghz dual core, 4 GB RAM). Could it have something to do with the USB or something?
Anybody else has this problem? If anyone has any idea as to what might be causing this, I would be much obliged.