I have a two-stage, 5HP air compressor with a 20 gallon tank. I am guessing the compressor is at least 15 years old. This has been a workhorse and has done me well until just this past week. It does not a on/off switch so I have to plug and unplug the 220. The motor runs at high RPMs like it used to and then the motor just get slower and slower until it looks like it is going to stop. I don't give it a chance to because I don't want to blow a breaker so I unplug it. The pump is full of compressor oil. I assume the motor is going out and starting not to develop enough torque as the tank fills. Anyone have any ideas of how to check this? Thanks in advance.