Did it do this before you replaced them? It could be the counter belt.
I took a look at the service manual, and I see a Hall sensor tethered to the deck circuit, so it could be similar to the M90's auto-stop sensing. If it can't find a spinning counter shaft, the deck stops. Notice in the diagram how the sensor goes to the "TAPE END" pin on the controller chip?
Check behind the counter and see if you can find the little PCB with IC902 on it, mounted against a spinning magnetic wheel or some such thing.