I have an identical problem with one '85 Okuma LB15. When RAPA stops counting, I have two situations:
1. Motor load over 200%, even without belt, until DIFF is too big and I got 100-4 Diff Alarm
2. Motor load over 200%, even without belt, but the motor at one point jumps (DIFF under 5mm), and I got a Zaxis servodrive error, and I have to turn off the lathe to get out of this error. So far, after this jumps, I had to replace two times a power transistor in the drive.
Until now, I have done all geeserteg's steps, and for the moment I'm scratching my head.
Any ideas?