Ford Repair: 96 Ford Windstar, 3.8L, ford windstar, listening device

Question
Just recently had new belt and idler pulley replaced due to loud belt squeak and pulley out of alignment.  A few days later, the squeak has returned and all other pulleys sound ok (with listening device).  Squeak is definitely coming from belt.  What can we do now to fix this problem?  This is the 4th belt we've replaced in 2 years and they haven't been worn out.

Answer
Aside from getting the right belt, the only thing I would check is the idler, make sure it is not binding and the spring is working properly.  Another possibility, there maybe some fluid spray from other devices such steering pump, water pump, oil, etc....  

Sam