Adding to gig229's comments, my Enclave had similar temperature fluctuations when it was down 0.5 lbs of freon. Fortunately a top up to the correct amount solved the issue.Our Enclave has a hissing noise coming from the ac system. When it hisses the temperature fluctuates from around 41 degrees and will go as high up to 58+ degrees. This is super frustrating. The dealer has already replaced the compressor and the expansion valve. Because the the GM engineering team can not identify this issue is out of any parameters, they are declining to assist further. They did admit that there are others like this to the service manager, but do not have a fix. I was told by GM customer service to find another dealer and see if they can identify anything different. The hissing and temp fluctuation is NOT normal. I can not see where anyone who had this going on would accept this as normal. Any advice is appreciated. I do have video recording the sound and temp fluctuation, but the file is to big to upload.
I concur you probably have a leak in the system. If it is the evaporator as gig229 deduced, that is an ugly replacement.
At this point, I suggest
(a) Trying a can of Freon / R134a (?) with Stop Leak in it. I have had some success with the A/C Pro products but there are many options in this area. If the freon leaks out again then you are out $30. Just do not overfill the system - a gauge attached to the "High" side will prevent you doing so. This typically helps with seal issues but a pinhole leak in the evaporate may be small enough to get plugged. The AC system needs to run and circulate the stop leak product so plan to run it straight for like 20+ minutes.
(b) Cleaning the condenser coils on the front of the Enclave. I use a spray can of foaming AC coil cleaner from Lowes. The foam soaks into the coils and helps get the dirt out. Will not fix the hissing but the AC will run more efficiently.