I would get a complete tune up. You are suffering from what 90% of drivers are. Neglecting their cars. A tune up should help if you get the complete tune up * carb adjustments, cleaning, regular things it should be okey.

It's possible also that the A/C clutch and or pulley are locked up and either not turning or turning with too much resistance. Like you said, only when turning the a/c.

Do the tune up first then check as follow.