Auto Air Conditioning & Heater Repair: Mystifying A/C Problem, head scratcher, chevy suburban

Question
I have a 1997 Chevy Suburban, which has had A/C problems off and on for the entire time I have owned it.  Two summers ago, my father (an experienced mechanic) and my husband started trying to fix it.  Basically, they have replaced every component of the system in the past two years, except the hoses.  Here is what the A/C does:  If the weather is cool, say, 70 degrees or cooler, the A/C will blow out of the vents at 30-35 degrees.  But if it is 85 or above, we get 65 degree air out of the vents (measured with a probe thermometer).  Since we were at our wits' end, I took it to a local shop that specializes in A/C, and was recommended to me.  The mechanic there worked on it for 3 days and could not fix it.  To his credit, he hasn't charged me anything yet, either!  To let you know, it doesn't seem to matter whether the engine is hot or not, just the outside temp.  When I drove it to the shop, which is more than 30 minutes from my house, very early in the morning when it was cool, it blew cool air all the way.  My father has a theory that perhaps a high outside temp causes the hoses to collapse on the inside, if they are deteriorated.  Before we replace all the hoses, do you have any suggestions?

Answer
Wendy,

Yeah this is defiantly a head scratcher and I don't think I have any helpful suggestions because you have already done pretty much everything to the system that can be done aside from the hoses. I haven't seen to many problems with a/c system due to collapsing hoses but it is a definite possibility now that everything else has been ruled out. Try the hoses, it couldn't hurt and if that doesn't work check the pressure of the system when it is hot and see if that made difference from before. Other than that I'm afraid I don't have anymore ideas for you. I'm sorry I couldn't be of more assistance to you.