Your vehicle isn't cooling correctly. It if ever hits H, then you've got a problem. Remember, with all mechanical things, heat is the enemy. I wouldn't worry about the A/C. When you end up with a warped or cracked cylinder head, then you won't have an A/C anyways.
Check and replace the thermostat. It's a cheap fix and a bad thermostat can cause all sorts of issues.
Flush the radiator completely and refill with a proper coolant-water mix. 50/50 is usually fine, unless you're in a colder climate, in which case you'll need to up the concentration of coolant to water
Given the time of year we're in, it's probably not freeze plugs. They're another cheap part that will go wrong when you don't realize it and suddenly you'll find yourself sitting on the side of the road screaming and cussing.
I'd inspect the water pump, too. The impeller might be shot.
Also, make certain that your fans are working, too.
Check your hoses and make certain there aren't any leaks.