When I would slash with the brush tool (to get those super-fine tapered ends) the thing would sometimes do something similar to what you're describing, it would automate as if I pressed down really hard the whole time. I'd have to keep ctrl+z'ing just to get what I wanted.
My course of action was to update the driver for my tablet, because I was pretty convinced it was the driver that was giving me problems. If you're driver isn't corrupted or out-of-date, it can't be the same issue as I dealt with.
