clean the coil and go from there if contactor is bad the unit wouldnt run at all if capacitor is bad it may or may not run but if it does it usually runs slow or will hum as it tries to pick up speed i still think its a dirty coil throwing pressure switch on a hot day
that'd be my first guess, You could probably spray it with water hose and get enough dirt off to get you by, I would do like gkelly said and clean coils and change cap.
