Auto or manual AC controls?
First make sure it has a good R-134a charge.
With compressor jumped and running post back you low and high side readings at idle and at 2K.
With a ac recharging gauge set.
Like one of these.
http://www.ackits.com/merchant.mvc?S...erantGaugeSets
Low side should be close to 30 lbs.
If it is dropping under 25 lbs the switch will cycle or shut off compressor.