Looking for auto a/c help

I have a 2003 Chevy Trailblazer LT EXT with front and rear a/c. The last owner said he replaced the compressor, condensor, canister and evaporator in the front. They all look pretty new. Orifice and expansion valve as well. It hasn't worked right since I bought it about 8 months ago. The weather has been tolerable up until now so it's issues are showing. It does have a very slow leak and I do plan on fixing that part soon but here is the issue I'm having:

At first I was getting all kinds of oil in the line on the high side so I drained the compressor of oil and started over with the right amount of oil. Put a good vacuum on it for a few hours and proceeded to fill with r134a. Took six 12oz cans to get it up to 180psi. The air coming out of the vents is only 75 degrees at coldest, which is only 10 degrees cooler than ambient temp. When revving the engine the compressor is blowing the pop-off valve like it's building too much pressure. The high side cuts out at around 190 and cuts back in about 170. I am using a set of Yellow Jacket gauges on a Yellow Jacket manifold which have been calibrated within the last month. I've been turning wrenches for over 35 years but a/c work is not one of my strong suits. Any ideas what the hell is going on with this thing?