Hi and welcome to JustAnswer,

Thank you for the question and your patience. Should you still need assistance...

Initially, the described symptom would indictae that the Nakamichi is kicking in its auto-stop mode; i.e. its sensors detects no tape movement, assume tape has reached its end. That it does this even when just about to play, rewind or fast-forward would infer that it is false sensing. This is then an internal electro-mechanical fault.

Unfortunately, no quick/easy fix and neither is there a workaround. The unit would have to be opened up for visual inspection, voltage measurements, confirm fault and effect repairs.

Offhand and after a review of the repair/Service Manual, primary suspects would be:

• weak reel or capstan motor (least likely);
• counter switch contacts (likely);
• loose counter belt (most likely).

• consider a DIY (do-it-yourself) repair if and only if comfortable working with electronic components & circuitry, use of a DVM/VOM, a soldering iron/station and safe practices with electrically powered devices. Access to a Service Manual or at the very least, a schematic diagram would be a necessity;
• should you be uncomfortable with a DIY, perhaps your best bet would then be to consult with and/or seek the services of an experienced Service Center or an qualified professional;