A/C intermintent 2nd time @ dealership. Help needed!

[firedawgs]

2009 35D w/ 10K miles.
Added OEM trailer hitch/electrical/software 4 months ago. Dealer installed.

I notice when it is hot and I take a trip (30 mins+), many times the air won't blow cold air. The iDrive is set to blue (cold). The blower is working fine. The first time it happened, I could pull over, turn off the car, pull out key, open door, then reverse cycle to turn on the car, just to have the A/C blow cold for maybe 5 minutes.

I have brought the car into the dealer the first time, just for them to tell me the iDrive was set to hot. I, at that time, believed them, and went on my merry way. I then drove the other day to my sister's house which is an 1.25 hours away. It was 85 degrees out, and wouldn't you know the A/C stopped blowing cold air about 20 minutes into the drive. Turning the car off and on didn't work this time.

I called the dealer right up, frustrated as all hell. I questioned if in the shop last time, did they pressure test the system, check for error codes, etc. Of course, they said yes. But, the fact of the matter is that the A/C blows no cold at warm temperatures and traveling.

I figure this is going to be a hard problem to determine, so I wanted to see if anyone had incite on this problem. I did read one post on someone having no A/C during towing, and I have added the trailer hitch (OEM) through the dealer install, but have not used the hitch yet. I have a appointment next Friday, so any information I can give them would be great.

Thanks for the help,
Sean