I have a '99 PS with the same problem. Has a Black Magic kit in it that used to engage around 4700, now is around 5200. Think that is clutch/belt issues. However, have the same problem where it just really seems to struggle to start moving. Seems to go just fine once it's rolling. Interestingly, when I pull the belt off and lift the rear then turn the driven by hand, there are spots where it turns hard and if I let go of the clutch it will actually turn backwards. There are other spots where it turns easily. Don't see anything funky with the track. I'll be pulling the chain case and looking for probs there and also checking all the bearings on the jack shaft and drive shaft. Will also look for bent drive or jackshaft and checking bearings in the rear idler wheels. I'll let you know if I find anything. If anyone else has solved this prob, let us know.