Problem with Peerless / Tecumseh 6-speed Transaxle?

I have Craftsman Model 917.270732 / SN:063000D010923 Lawn tractor. It came equiped with a Peerless / Tecumseh Model MST-205-545C / SN:0173B-0751 6-speed Transaxle. The tractor does fine in first, fourth, fifth, and sixth gear. However, in second and third gear the tractor jumps and then slows back down in somewhat of a predictable jumping pattern.

Belt was replaced with a Craftsman made belt and although the tractor has some age on it, the pulleys and idlers seem in good shape. They are not bent or warped. I have looked into th transmission and did not find any chipped gears or visually identifiable faults with the shafts and other components.
Anyone have any suggestions? I cannot afford a new transmission or a new lawn tractor.