2003 Pontiac Montana Rear brakes again?

On 10/19/05 at 32498 miles, the dealer had to replace the rear wheel cylinders, due to leaking fluid. This was done under warranty. The brakes started grinding really bad and sound like a school bus stopping, and I took it to my regular shop, and they saw what it was and sent me to the dealer, since they were under warranty. Shoes were contaminated and they had to re-build it all and perform a rear brake service. On 10/14/06 at 44715 miles, rear drums had to be re-surfaced again. Now they are needing it again at 62000 miles. The grinding started about 2 months ago. And it's only when it's cold. As soon as everything heats up it stops. Again. What in the world could be making my Montana need a rear brake job so often. I've needed ONE brake job on the front to the 3 on the rear!
Thanks much!