took it apart to look for obvious component failure or mechanical damage in the motor drive area. didn't find that. what i did find was a tiny little ball of solder that had somehow managed to get trapped by a wire in between two pads which if bridged would tend to pull the motor speed up and interfere with servo operation. since it was an intermittent problem i'm not gonna say for sure that it's fixed, but i remediated a fault that i would expect to cause this problem and cause it intermittently
i was giving it a test with morgan's standard reference cassette (see photo) and i shut the lid and the speed control immediately went out of detent and jumped up about a minor third. i know from experience that just means the speed control pot for quartz lock mode wasn't set right so i must have had it right on the edge of the "acceptable" range and slamming the lid was enough to bump it from good to no-good. opened it back up and reset that and it's back to fine