You got me..Capacitor? where? At the start switch? Ill try all the suggestions tomorrow am.
Most air compressor motors have a start/run capacitor to help the motor start spinning. The description you provided leads me to believe the start capacitor is bad. I've encountered and repaired this problem many times. Judging by the pictures you provided, the start capacitor appears to be the long black cylinder located under the compressor motor, the one with the yellow wires coming out of it.
When replacing the start/run capacitor, it's important to replace it with one of the exact same type and rating. In most cases it's ok to go a little over on the microfarad (uF) rating but never go below it or the voltage (V) and temperature ratings.
Lastly, I second the advice about not running an air compressor using a long extension cord. The resulting voltage and current drop is simply too much for most air compressors to start and/or run properly.