Generally, temperature creep while sitting, idling, is insufficient air flow through the radiator. Are you SURE the fan clutch is working properly? It should be noticeably noisy, like a wind tunnel, when the fan clutch is "clutching up" when hot. At 200 degrees and hotter, the clutch should be running at engine speed. Put your hand behind the fan, engine side with it running hot. You should feel a tremendous amount of air being pulled through the radiator. Possibly the fan that is installed on the clutch is not made with the proper pitch. (possible, but unlikely). Do you have the proper size pulley on the water pump? Possibly they are not turning fast enough at low RPM's for the AC system and pulling enough air past the condenser and through the radiator. I would start in these areas.
One thing I would caution you on would be to jump immediately to an electric fan system to fix this problem. In working on these cars for decades, I rarely find ANY electric fan system, single fan, dual fan, push or pull style that will out cool a proper clutch fan system. Good luck with it.
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