Where is the "crunch" coming from? Also, did you clean the thread tension disks? They can get gummy. My two 500's use a bit less tension in the bobbin case than we're use to on most machines... My last thought is that you have binding somewhere in the upper mechanism. On rehabing a machine, I am very liberal with the WD 40 until it is dripping from the bottom if the machine was bound up, just to insure that all parts are free to move.