ok... I might be reaching here...
did you recently remove any of the arms?
I'm wondering if maybe you forgot to replace your caster adjustment shims.
Or if you changed the caster adjustment at all. You should have two shims in each front upper suspension arm.
If you have not messed with them, maybe try adjusting them to have 2 shims towards the rear of each arm. (they go on the inside to push the arm as far back as it will go)
...this should increase your caster angle to 10 degrees, making your truck want to stay in a straight line more. Also give more traction to the front wheels etc...
Stock is 1 front 1 rear giving you 7 deg. if you put them in the front you get 3 degrees
If you don't know what the heck I'm talking about ...look at pg20 in the manual;
http://www.traxxas.com/PDF-Library/3905_manual.pdf
...they explain it better than I can.
its traxxas part # 5134
anyway....what i was thinking was that if you were missing them...maybe when you accelerate the arms are sliding around on the pins and messing with your steering control.