Have 4.6 with 205,000 miles. Uses 1 qt oil / 4000 miles. Rebuilt transmission. Driveline is solid and tight. Recently replaced A/C clutch assy and A/C works great normally.
When towing (2000 lb boat and trailer), the A/C cuts out at the SLIGHTEST grade. It used to cut out, but only on a more serious grade. Any Ideas?

 

Is it cutting out or is it reverting to defrost?
If going to defrost check vacuum check valves, if they are good use a hand vacuum pump and check for leaks