GMC Envoy A/C Help

We have an 08 Envoy. A/C went out a couple years ago. Replaced compressor and condenser (a mechanic did it). Worked for a while, then intermittently, then stopped working altogether. We just replaced the coolant temp sensor (that job SUCKED!) and filter and now it works decently well again. Refrigerant was at the correct pressure. We added just a small amount but didn't overfill. The A/C works but doesn't consistently blow cold air and the car starts to overheat when parked. Anything else we can do to make it work any better?