We have seen this problem on several of these units. We started adding a taller pulley to replace the rear idler and it seems to fix the problems. If you want email my service manager Craig
and he'll explain the process.
As well Hustler has warrantied every belt and pulley we have installed. It cured the problem and we now install the pulley on each unit BEFORE it is sold. The shorter you cut your lawn the worse the problem seems to be.