Need advice about VCR to PC connection
The problem is almost definitely with the tape or the VCR. Most likely the sync or black level is off or the control track is fluttering, which is throwing off the ADC in the capture dongle. You will need a time base corrector (TBC) or frame synchronizer to correct the problem.

Ideally, you should also have a way to view and adjust the video levels. Usually you can adjust levels with a good quality TBC or FS. You can use software to view a waveform and vectorscope after digitization.