AC stops working and starts working

2003 Merc. SABLE. AC works great for 1/2 hour on the highway. Then No cool air. We shut it off. An hour later and 70 miles down the road , it works again. Always cold air… no matter where you set temp. 4 times to 3 mechanics. No one can solve it. Please help.

By no cool air do you mean that you don’t feel any air coming out the vents or the air coming out the vents is not cool, but it is moving?

Keith. Almost no air coming thru vents. What we do feel is warm.