Nissan Repair Questions? Ask a Mechanic for Answers ASAP
It sounds like it could be as simple as low Freon, especially if there was some body work in the front done in the past.
Sometimes we see that the system will develop very small leaks and then needs to be refilled after a couple of years. Most likely the leaks won't even show up on a leak test because they are so minor.
Typically they are pretty good. Out here in the southwest we don't see many issues with them.
Did the shop test the pressure readings in the AC?
I assume the shop is fairly well qualified, but it's not terribly difficult to test whether the problem is a faulty AC compressor, low freon level, bad pressure switch, etc.
My bet is still on low Freon levels, otherwise it wouldn't work at high speeds.