I'll assume that it is a variable speed drive system w/ 2 traction belts. The full model # could be helpful. Usually that indicates an issue w/ the front traction belt. Can be a highly glazed drive pulley, wrong belt, etc. The belts are meant to be replaced as a set BTW. Always use the OEM belts as well. I've seen this condition when only the front belt had been replaced. Rear only belt changes one can get away with for a time or two before replacing the front.

Another problem with that type of drive: The sliding sheave on the double stacked pulley sometimes gets rusty, and doesn't slide up and down like it should.
A word to the wise-If you don't have the patience of Job-send it out for repair! I spent two weeks on one of mine, drove it outside to warm up! The "warm-up Gremlins" must of been mad at me, because it got really hot, and burst into flame! Burnt both new belts, and everything else! RJ