How old are your SS springs? I had a very similar thing happen, the engine ran fine but it kept going slower and slower. I began to break things like u joints, the yoke on the rear end, and even the 65 only extension housing. Someone finally told me that they could see the entire top of the rear tire when I launched. He suggested that I look for "witness marks" on the front of the rear shocks since the rear end wrapped up so much. The shocks were almost smashed. I would have never believed the springs would allow that, but it sure did. After speaking with some stocker friends, they said as soon as the 60's begin to fall off, get new springs. Of course, that was in the days before caltracs.