A/C can only run by itself, anything else trips it

Bought a VOM and output if I am reading it correctly was 135 (way to many options at the electric meter section as far as I'm concerned too.) And AC unit says 120V inside the cover. I can flip the 20 amp circuit that says a/c on it and only the a/c unit turns off. So when everything shuts off it's the main breaker. Do I replace the breaker? Replace A/C unit that is almost 10 years old? or Switch to 50 amp to get the extra juice I need and put in another a/c unit? Or all of the above.