I had a similar problem on an old Audi 90 quattro.
The problem was hanging the exhaust so the weight distribution on the hangers was even, but the exhaust pipe was routed over the rear differential, so this proved almost impossible.
I tried polyurethane hangers and these broke too!!
I would check that the exhaust is hung properly first, if the problem persists, do what i did with the Audi, come service time, fit some new hangers, they are not expensive.