I've had the same problem (more or less) on my 99 600 PS. Haven't tried pulling backwards, but does it pulling forwards and sometimes throttling forward it seems like the brake is hung up. I'm convinced the problem is the track has precurved itself from sitting. This is my backup so it sometimes doesn't really move for 8 - 10 months. While I store it on a stand, I don't de-tension the track. To test I put the sled up on stand, took off the belt and turned by hand. Would turn OK then turn hard. Marked the hard spots and they repeated. I'm convinced those are spots in the track where it curved around rear wheel or drivers and has now taken that curved set. When it drops that spot back around the wheels it doesn't like to move from that spot. I can even see it in the track some now that I know what I'm looking for. I assume some of it is the track is now 10 years old and is losing some flexibility. Figured if I ever got it out on the snow for a day or two and got the track warmed up good that it would straighten itself out. I am more careful now when I put it away as well. Have found that if you spin the track on the stand it will almost always stop at the same spot. I now make sure it doesn't. I could be all wet, but that sure looks like my issue.