how does the window motor know when to shut off?

The rear driver's window on my '81 300TD has some odd habits as of late:
1. it slips down so that I start to hear a whistle through it
2. it "thumps" rhythmically when going up or down
3. when all the way up the motor keeps going resulting in a thumpthumpthump until I let go of the button.

I had the door apart tonight and tried to visualize what was going on...I have experience replacing the front regulator on my car...but the regulator is tucked away and I couldn't extract it even after removing the retaining bolts. I did not visualize any breakage anywhere on the regulator.

what gives?