300E---sporadic starting problems

Just recently, my 89 300E failed to start after driving for several miles. The air conditioning was on, and I noticed at stop light that the air seemed to slow in it's power----well when i cut car off at the office it would not start. I jumped it, and it started immediately. Took to service center----and belt was loose----mechanic stated the belt tensioner needed replacement---did so----however today--just one week later cut car off---and would not start again---would not turnover----jumped it started right up-----Any suggestions what this might be---just invested 569.00 for prior repair and it seems problem persists.