96 Tracker timing

My Tracker has problems falling out of time.

My mechanic has replaced the timing belt first, without success, then found a worn keyway in the timing belt gear on the crankshaft. Replaced the gear and key, still idled rough which was fixed by installing a used distributor.

Now 2 months later the timing has been lost again and I suspect the new belt has again slipped a cog. Seems as though this may indicate a worn tensioner on the timing belt.

I'll take it back to him again to check into it. If the belt has slipped again on the cog, could this be related to the tensioner, or is there possibly something else that could cause this reocurring problem?

Thanks in advance for your ideas and suggestions.

Ed..in Va.