Why does my air conditioner blow hot air under load and blows cold when not?

ck freon level and pressure you may have a restriction causing high pressure at cruize

Sounds like a vacuum leak.See if air is switching to defrost when going uphill.If so there is a vacuum problem. Look under hood vacuum tubing is small plastic and looks almost like a wire.Look around with a light and check to see if tubing cracked broken unhooked ect.