The rubber “idler” is slipping. Also the “stop” mechanism is based upon the tape actually moving. If the mechanism doesn’t detect movement, the unit will shutdown to prevent the tape from stretching. IRRC: This is accomplished by a “feeler” that will trip when the tape tension gets too high. This unit needs an overhaul.
It is possible that the motor is getting weak, but I would start with replacing the idler assembly or at least the idler tire.
Note: given the age of this, some of the lubricant may not be lubricating so well. Over time this will become more like glue. You may need to re-lubricate the assembly as well.
Dan