HP 67 Circuit Board

[metawops]

Today I had a little time and made a few tests. Turns out that the motor sometimes starts but sometimes not. More precisely: it starts only the first time after applying power to the calculator. I was able to take a video of the current „state of the union“, please find it here (~176MB).

Two obvious things:

1. The sound is irregular so I assume the force the motor tries to apply is irregular, too. Maybe the reason could be a not perfectly round replacement tubing?
2. When trying a 2nd, 3rd, 4th, … time we see that the motor *tries* to move but it can‘t. Why did it work on the 1st try then? And what keeps it from spinning up after the first time?
   

Any advice from the experts?