The outlet is fine. I have run compressors, electric mowers and table saws off the same outlet. It has a 15Amp GFCI also.
I just took the unit apart (which was a bit of a chore). Lo and behold! The circuit breaker that was attached to the side of the motor is rated at only 5 Amps! This has to be the problem because the unit is advertised as having a 13.5 Amp motor.
I can't find any circuit breaker specs on the web and the manufacturer is closed today. I might just see if I can find a 10 amp breaker and try that. I will report back with results next week.