If it is only slightly veering, it may be because you're using old motors and motor-bias is getting to you. If that's the case, fiddle around with the encoders in your code for scaling that'll decrease/get rid of the problem.
Check if the two CIMs on both sides are powered. You might be powering just one on one side and two on the other.
Lastly.... check if one side's chain has too much tension. The tension might make it harder for everything to turn on one side.
We had this problem too. We propped it up on some blocks so the wheels were floating off the ground. Eventually we found out it was a programming error, but it's a lot easier to diagnose the problem when the wheels are off the ground.
EDIT: Also check your gearboxes. Clean the debris if there is any inside. We had a problem two years ago when the gearboxes were mysteriously locking up (we were using toughboxes). What we discovered was that one side of the gears has a little "lip" machined onto it to prevent it from rubbing against the plates. The lip needs to face the plates.
See here. The side facing us right now has the "lip."
Post back tomorrow when you run the tests!