Thank you for the inquiry.

If you do happen to have the model# of the unit that will be helpful as I'll be able to give you more accurate information with the model#.

Regardless, there may be a few potential problems causing this. I would first confirm your tank Check Valve doesn't have any debris inside (from the head, possibly, or condensation build-up) as this will cause air to force its way into a smaller area which could bog down the unit.

Also, make sure the Belt is not too tight and that the motor pulley and pump flywheel are aligned as best as you can tell. As far as belt tension is concerned usually you want 1/2" deflection, when you push down on the belt. This seems obvious but is pretty commonly overlooked.

Lastly, I would say that it could potentially be an internal pump problem whether it be the crank bearings or connecting rods seizing. As long as you have a straight 30 weight, NON-detergent compressor oil in the unit and the oil level is sufficient, this should not be the problem.

I've attached a couple of documents where you can find additional trouble shooting information on these oil-bath electric compressors that may help you further.

I hope this helps as a starting point, good luck and feel free to post the answer to the problem in this forum if you find the solution!
- MTR

Sounds good, looks like you have most bases covered.

The main brass check valve in the tank should probably be removed, just make sure you drain the tanks completely prior to removing it. It's like an iceberg, you'll just see the large hex fitting externally, the valve is in the tank.

Drain the tank(s), remove the main Discharge Tube from the pump and remove the Check Valve to see what you can find.

Lastly, you may want to confirm your capacitors are in good shape on your motor. If the Run Capacitor has lost "juice" it will not provide enough energy to keep the motor running at higher pressures. Let me know what you find.
Thanks again!
- MTR

Also, check the Reed Valves in the pump head. Many times the reed valves will rust and stick to the Head or Valve Plate (if it has a separate valve plate) which will cause this problem. This is something you'll definitely want to check out.
- MTR