Air Conditioner Problem - 2001 Expedition

I have a 2001 Expedition with the smaller V8 (4.6L. I think).  When I have the Air conditioner on and I accelerate, the air switches to the defrost mode and is warm.  Then when I go back to cruise mode the A/C comes back on cold.   I'm baffled by this. 
 I had a tune up about a week ago, so I'm thinking maybe a vacuum hose was left loose.  I returned it to the shop and they said they couldn't find any vacuum leak.  They now want to charge me $79 to do a diagnostic and find the reason for this problem.
 
Any ideas? 
 
Thank you!