Serpentine belt tensioner is jumping and acting weird

This is an ongoing problem with my '93 F250 (7.3 IDI). The stock length belt does not result in the tensioner staying in the middle of its range after a few minutes of breaking-in a new belt. One that's half an inch shorter (I think) lasted a while.
Now it's chirping again. As pointed out above, if it's too loose the tensioner can't keep it tight all the time. On my old truck there is also plenty of oil and other fluids around, some of which undoubtedly get on the belt. Once there's a glazed spot I think it will always chirp.