Yep, need gauges on it to really tell anything. Is the compressor running when it's not blowing cold? Look at the center of the pulley to tell. The outside part always turns when the engine is running.
It could be the radiator fan not running causing it to trip on high pressure, but I'd think you'd be overheating too if that was the case. Plus, once you're driving down the road, you don't need the fan anyway. It sounds like the compressor is being kicked off for some reason. It could be an over/under charge, defective high/low pressure control, temp controller problem, etc.