Strange behaviors like that are common for Beogram 8000 and 8002 turntables that need restoration. It’s difficult to say offhand what the fix for the symptom is because there can be several problems going on. The problems result from bad information from failing circuits reporting what is going on back to the Beogram microcomputer IC.
For certain you want to begin with replacing the old electrolytic capacitors.
While doing that you should reflow solder joints of all the board connectors.
Once those things are done you can retry and see where your Beogram is at.
It is a good idea at that point to measure all of the key power supply voltages and perform the service manual adjustments.
I also like to check the health of the sensor signals (using an oscilloscope).
Those steps should catch any problems.
I hope you sourced your servo belt from Beoparts.
-sonavor