Hi, I used to have the same problem until I got a bass teacher who showed me an exercise that cured the problem for me. I have described it in the thread "How to get even plucks?" in the technique forum. Read the whole thread, it should help you out. Think economy of motion.
My speed limit is now 16ths in a tempo about 95 bpm and I can hold that at least a long as a normal song lasts. It's still not that fast, but it was a major improvement for me.
EDIT: Forgot to mention this: Play as lightly as possible. Also on gigs - Many people, including myself, tend to play more aggressively on gigs than while practicing.