80s Funai-made VCR, erratic speed in SP mode?

[PhilcoSearch]

I've been working on a Multitech MV-080, which is a 2-head HQ mono VCR that was made by Funai in the mid 80s. There are several functionally similar or rebadged versions of this same unit inlcuding the Symphonic 5200, Teac MV-350, Funai SVG-2000, etc., so hopefully someone on here has seen this problem before.

It sounds like the drum motor speed is fluctuating up and down regularly in SP mode, whether recording or playing. When playing commercially released VHS tapes, the picture comes up for half a second and goes noisy in a regular cycle and the audio is slow and warbly. It will record in SP and play it's own recordings normally, but they display the same periodic noise when played on a good machine. It records LP and EP (SLP) correctly and those recordings play fine on a known good VCR.

I tried a couple fixes already including replacing the pinch roller. I'm just a hobbyist, but using a service manual from one of the clones of this machine, my best guess is that one of the ICs in the servo circuit has gone bad. There was actually a handwritten note in the manual from a past technician about replacing one particular op-amp IC to solve a similar speed problem, so I found a replacement and tried that to no avail. I thought about getting a parts machine to try replacing the more hard-to-find ICs through trial and error, but I wondered if there might be a simpler explanation? I thought maybe caps, but the few I tested were still good and the power supply voltages are fine. Any insight would be appreciated. **Edit - I should mention I did replace all the belts as well.