Sounds like the AC might be running hot and going out on overload. Check the breaker for the AC, breakers do get weak as they age or a wire loose, if not then check some of the other ideas in previous answers, I always check from the power to the unit that does not work. also if you are using an extension cord it might not be heavy enough to carry the AC which will then overheat and go out on overload, when checking for voltage drop or amps the unit being tested should be running. Good Luck and Happy Camping