The problem is not that the tensioner gets loose, but that it takes a "set". The tensioning medium is rubber and after prolonged exposure to tension and heat over many years the rubber does not rebound fully anymore. When the center bolt is tight it will always be difficult to swivel. The thing to check is the unloaded resting position of the idler wheel relative to the machined flats on the center hub on the back of the unit. The best way to do this is to take your old tensioner in to a dealer and ask them to pull a new tensioner off the shelf so you can compare them. If you have to replace it FastLane will have a much better price than the dealer.
Read this earlier thread where I've included some pictures explaining what to look for:
1993 300se belt tensioner adjustment
Hope this helps