2003 gmc envoy blows warm air. Couple years ago it took the coolant and worked fine but now gauge from the coolant bottles needle is jumping around but then hangs up in the alert range. The compressor kicks on but almost kills the engine each time it does. Any ideas what it could be?

High pressure valve or a bad compresser

You may have it overcharged. If theres too much refrigerant in it the low side pressures will be too high, and the compressor can be sucking liquid refrigerant instead of gaseous, and it'll "slug". It's really hard on the compressor. Those parts store cans of refrigerant at the parts store are huge, most cars today don't take a ton of refrigerant. It's really pretty common for people using those parts store cans to overfill. Get it sucked out and refilled properly. Start with a known proper charge before delving into it further.