2002 Buick Rendezvous AC problems

This Buick has 46K miles and besides the AC problem that I am experiencing now has performed flawlessly. I have taken this car to the mechanic to get the AC checked out and what they told me was wrong, was the AC compressor but they said it would cost $1200 to fix $700 for the parts and $500 for the labor. I got a 2nd estimate with similar rates but was told I could by it myself for cheaper and they would put in it for me. I was able to buy the part myself from advanced auto parts and get them to put it in for me. It took about 2 hours for them to put it in and the AC worked for about 2or 3 days and then began to run hot. I took it back and the mechanic said it could possibly be a leak in one of the pressure valve. That was replaced and two cans of Freon were added. Again it worked for about 2 days and began to run hot. I took it back again and this time he suggested that it could be the evaporator coil? He said it could possibly be fixed by putting evaporator leak stopper. We loaded that and put cans of Freon and it stayed cool in the car for about 2 ½ hours. It is now running as if we never did anything to the car. Please help?