The machine works, but shuts off after a few minutes. The setting makes no difference; the dryer hums when the start button is pushed; the drum can be rotated eaily counterclockwise, but it is very hard to turn it clockwise; the belt is not broken, but it is starting to fray; once started, it gets noisier then shuts-off after a few minutes and cannot immediately be restarted. Any thoughts?

the drum can be rotated easily counterclockwise, but it is very hard to turn it clockwise;
The question is what is the normal direction of the drum. Check it in that direction. If it is hard to turn track down what is causing/jamming the drum.

Check the motor for bearing slop and give it a spin to see if it rotates freely.
heck that there is nothing in the blower wheel.

shuts-off after a few minutes and cannot immediately be restarted. Any thoughts?
Your motor is overheating for some reason.
Could be the motor is toast as gabe_1981 said
Could be that there is too much load on the motor.
Could be that the motor is packed full of lint.

If it needs a motor I would also check all other moving parts, front supports, drum seals etc., best to do a bit of an overhaul on the unit. This will give you a total cost and then you can then decide if it is worth putting that many dollars into the unit.