3 possible reasons:
1. Your video head needs cleaning;
2. The pinch roller is no longer making full contact with the entire tape width; or
3. Your capstan motor is running out of standard speed.
Corrective procedures would call for:
A. Clean the video head with a tape head cleaner;
B. Clean, re-align or replace the pinch roller; or
C. Re-calibrate the capstan speed. In some models, there are 2 adjusting trimpots for SP, LP and SLP speeds. This procedure would call for a special tachometer and/or an alignment tape. In the absence, you perhaps can try winging it (adjust till you are satisfied and no picture roll). To determine which are the trimpots, you need a service manual or at least a schematic/parts placement diagram. Further you need to be familiar with electronic components and circuitry as well as use of a DVM. of course this would involve running/diagnosing/adjusting with the case/cover open and the unit plug in to the mains and running. This would also call for extreme care working with live units.
Should you not be comfortable doing a DIY, perhaps your best option would be to seek the services of a qualified VCR technician.
Hope this be of help/idea. Pls post back how things turned out or should you need additional information.
Good luck and kind regards.