I agree with Bob - sounds like the unloader is not opening when the pump shuts off (your 5,500 watt generator should be able to start the compressor with ease).
Note many of those direct drive compressors actually have TWO unloaders:
One is the traditional unloader valve incorporated into the pressure switch. It opens and bleeds pressure off the pump when the pressure switch opens. All small compressors have this one - it's what makes the "psssht" sound when the pump stops.
Some compressors have second "starting valve" teed into the compressor discharge line. This valve is normally open. At startup it allows the motor to spin several revolutions (build RPM) before closing and loading the pump.
If the starting valve is stuck shut it makes the compressor draw excessive current at startup making it hard (or impossible) to start.