Overheat issue

Ok i have a 90 with the closed system and as I am new into the XJ I am unsure of where to go with this. When I am up here in Apple Valley I stay at 210 deg constant no matter the terrain I am on...no overheat. If I go down the hill to like Fontana area i am constantly overheating. I was told possible air in the system and i need to switch to a 91 radiator and overflow bottle that is a vented radiator. this makes since but i wanted your guys opinion on this. I am currently running 2 electric fans as i eliminated the manual fan and clutch. Any help is appreciated

I have a three core rad, flowkool WP, ZJ fan clutch and I run water wetter. In Riverside It gets to the quarter mark. Out here about 200* (about center).

You don't need to switch. You can "burp" the system. When a closed system is working well it works very effectively! Do a search on here for "burp closed system". There are a ton of threads about it.