Hm, sounds a little like my recent encounter. In mine it was a huge friction build-up on couple of pulleys the belt runs on. A roller and the AC clutch were the culprits. When starting it would turn, but sort of choke at the same time. It did it a few times, then got better, then ... just did it a lot more when it eventually started, but one roller was completely seized and caused the belt to break, which finally lead to finding the problem.