Have you hosed out the fins on the rad & a/c condenser? You may not be getting proper airflow through them when the a/c is on. Wash them from the inside out with a hose. If a 99 is the same as mine you can take off the upper fan shroud to have better access to the a/c condenser.
Still could be an air bubble. Are you sure there isn't any air in the cooling system? Take it to a shop that has an evac system they can hook up to the rad & it sucks out any air in there. Or drive the front right tire up on something so it's elevated at least 6" then take the rad cap off & rev it around 1000 or so for a few mins or untill it gets to operating temp, top it up, put the cap back on & back off the block or whatever you used to lift the front.

 

210F is not overheating. With a 195F t-stat, most vehicles run right in the 200-210F range as indicated on the factory dash gauge.

The system shouldn't really change temperature once it's fully up to temp though regardless of load and/or a/c usage. If it does, that indicates a problem with the cooling efficiency under high thermal loads. Things to check which have already been mentioned; fan clutch, rad cap, and exterior of the a/c condenser & radiator. An inspection of the interior of the radiator should also be done to make sure that all of the tube ends are open if everything else checks out.