I have/had the same problem with my 2006. I'll explain.
I was having the same symptoms early in the summer. I took it to McCoy Freightliner in Portland. They emptied and recharged the system and gave it back to me. It worked for a while, then started working intermittently as you described.
I took it back, and the tech spent a lot of time tracing the problem. It turns out that sometimes the a/c compressor clutch won't engage. Their computer shows a short at the clutch (I may be using the wrong words or giving a bad description). They clear the code, and the code comes right back. They said that replacing the compressor might solve the problem, but couldn't be sure. I decided to just drive it, and then decide how big of a problem it was. Interestingly, it's been about 3 months, and the a/c has worked flawlessly, so obviously I haven't done anything about it.
I've been wondering if the problem is somehow tied to not using the a/c all winter. I have now learned that the a/c should be cycled on once a month, and will do that this winter.