summers

I have a 2021 GMC 2500HD Denali that stops blowing cold when reaching highway speeds. When first start it and idling, it will blow cold. Usually as long as staying within the neighborhood, it will blow cold; however, once get onto the highway and over 55, it stops blowing cold. I can put the truck into neutral to coast on the highway, and the A/C will usually begin blowing cold again. However, compressor shuts off once I put it back into drive. I have had the vehicle at the dealership 3 times over for the past 3 weeks with this issue. First visit (6/17/2024), they reprogrammed the computer (not covered by warranty, so $630 bill), and I was able to pick it up at the end of the day on Friday, 6/21. Second visit (6/24/2024), they replaced the compressor ($1600 bill as "only partially covered by warranty") Picked it up on the Wednesday, the26th, but wasn't able to take a highway trip until Saturday, 6/29 where I still experienced the issue. Third visit (dropped off on 7/1), they called me to say they had no clue as "computer shows no issues" but I needed to come in to disconnect my Banks Derringer as they "were certain" that was the issue. They "drove it a few miles" afterwards and said the Derringer was the issue all along as "the A/C worked like it was supposed to." I picked the vehicle up on 7/2 and left town on a road trip on 7/3. As soon as I hit highway speeds (Derringer still unplugged to prove that was not the issue), the A/C stopped blowing cold yet again. Further info., the technician/dealership has not yet opened a TAC case - unofficially due to the technician's ego. Anyone have any ideas/advice as the dealership cannot seem to properly diagnose the issue. I have another non-refundable camping trip this coming weekend, so I need to get this issue resolved... Thanks!