What radio system are you using? Because I know that some receivers don't work well with high powered servos, such as the savox you have installed, and need an additional glitch buster to work properly. I had a similar problem with my savox using certain spektrum receivers. It would shimmy side to side (never centering) and then would not steer linearly at all. Once the glitch buster was added it was all okay. Sounds very similar to what you have described.