Here is where a model and serial number would be very helpful, so we can see what deck you have and where the possible problem may be.
Make sure to use an OEM MTD belt if not using one now. I would start by making sure there is no grass buildup on the deck, and make sure the pivot arm with the idler pulleys on it isn't binding. Sounds like the arm may not be moving all the way and not fully releasing the belt.