1998 Toyota Camry Car runs hot, unless A/C is turned on.

Hi, recently I noticed the thermostat gauge show the car running hot, not overheating, or in the "red area" near the H. But if I have the a/c off, it starts climbing higher, and if I turn the A/C back on, the temp is right back to normal. I took it to a mechanic, they checked the radiator fluid, it was fine. They also made sure the cooling fans were turning on at appropriate times, and they were working fine. They couldn't figure out what was causing the high temp to go down when the a/c is turned on. I'm thinking maybe there's a ground wire or relay issue?