six axis walk/run detection

i have a six axis controller that was appeared to be working fine until i started playing assassin's creed 2. as everyone knows, when you lightly push the left analog stick forward he will walk slowly. the more pressure you place on the stick in the forward position the faster he will walk. with my controller he will studder step a lot. in other words if i'm holding r1 trigger and x to run he will start and stop a lot. it is very annoying. anyway, is it because it's a six axis instead of a dual shock or is the controller broken? if it's "broken" is there an easy fix or do i have to buy another controller. i love the game, but this is making it hard to play at time.