mtd servistar 18hp 42" lawn tractor?

You may have damaged the belt by lowering the cutter at high engine revs. Obviously with the engine turned off and key removed check the belt for damage as it will stop cutting in long grass at the damaged point.
Fix by purchasing a new belt, then only lower / engage cutter at low engine revs, once cutter is turning then increase revs. This will help stop belt damage that causes belt slip. The belt auto tightens as you lower the deck.

Trevor
May 2009