I picked up a Sanyo Betacord 3900II at the local flea market for $10.. I got it home and loaded a tape, and played rew and ff perfect with no worn belt issues.. Appears to be hardly used and there is no corrosion and no dust in the machine, almost looks new.. I noticed when playing a tape the head motor keeps shutting on and off.. I thought maybe a bad electrolytic maybe the issue, So I replaced a few of them on the servo board, where the head drum speed circuit is located and did no good.. I tapped on the circuit board, seem to not make any difference, I also tapped on the head drum and moved wires around and also seem to make no difference.. I was wondering maybe someone would know what would be causing this? Of course when the drum is spinning around the correct speed, the picture will appear until the heads slow down to a speed that's unreadable..
Here's a video that shows the problem:
http://www.youtube.com/watch?v=iJek56O2GsU