The thought I keep coming back to with your symptoms is to check the power supply voltages when the machine is in record mode...I believe the servo system operates on a +15V power rail and the logic off of a +5V power rail...audio is +/-15V I think...but I have had *really* weird inconsistent unpredictable things happen when the logic rail is failing or being pulled down below functional threshold level by some downstream failure. So identify what each of the power supply outputs are supposed to be, and then measure them when the machine is operating. That's what I would do. If you choose to do this, be very careful...triple check your actions before you do them...milliamps can kill...careless errors can drastically roach your gear. I'm not responsible for any negative outcome if you dive in, but I know that's what I would be checking.