my guess the Pin and bushing that the idler arm pivots upon are wore out? i just replaced them on mine this spring.
i have begun buying my belts from Shoup, their Q Power belts.........and so far they are working well. most of the large belts are made by Goodyear and run 1/3 to 1/2 price of OEMs. this vari speed belt on the feeder is the one i started with and so far it has held up very well. and regular ground UPS service with Shoup is better than 2nd day delivery with anyone else i have found.
Jaymo is right in that the pivot mount either breaks or bends. This is quite common. Solution is to double gusset the pivot mount when you have the variable pulley off. Run one vertically just ahead of the stone roller slide. Run another one forward to the vertical tube reinforcement of the feederhouse.