VA-DC Coaches. Hoping someone can help us.

Our robot normally runs very straight. But lately we have been noticing a couple of spots on the mat where our robot will almost always do a short jog to the left of about one cm. The spots are very specific and the sidestep happens with near 100% regularity. Everywhere else on the mat, the robot drives perfectly. We have tried cleaning the wheels many times. The mat seems clean, but there are some wear marks in the problem areas. It sure doesn't feel greasy or slippery or tacky in those specific areas. I am at a loss. I am hesitant to try cleaning the mat with anything, but I'd like to hear if any of you guys have any ideas.

To be clear, it is a very specific spot. We can program in a move tank block, 10 revolutions, and aim the robot to pass over the spot. No matter how far away we are at the start, it will shift at that same spot.  

Coach Skip Morrow