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.