timing belt issues

I have an 86 trooper and my timing belt broke, bent all my valves, etc. I had the cylinder head rebuilt, all new belts put on, new gaskets, etc. Less than 100 miles later my timing belt broke again. I had them check to see if a pulley was bent or something and all checked out ok. Other than a bad job by the mechanic, or a bad belt, is there maybe another thing to check which may have caused it to break again so soon? thanks