Running Hot

I have a 98Xj Auto, it recently has started to run hotter than normal, but the odd thing is that it only does it on the Highway. It doesnt get to the point of overheating but is just some cause for concearn for me. I have installed a separate tranny cooler, new radiator cap, flushed my coolant system and added "Kool-it" to the antifreeze and it all helped to lower my temp but only by a couple degrees, but on a hot day going down the higway it was have no problem hitting 220, and it has never done that before. But if I am sitting in traffic I can have the AC going and everything on the hottest day and it stays right on 210 never goes any higher. Has anyone ever experianced anything like this? I was thinking that it may be the transmission running hot, I did install the cooler but it is in conjuction with the factory cooler inside of the radiator.

If the cooling system was flushed then there are only some other alternatives. One would be the water pump, the vanes in the pump may be eaten away. The thermostat is not opening fully or the spring in the lower hose has been corroded away. I just pulled and replaced my hoses couple of weeks ago and the lower spring was pretty nasty looking, it came out in pieces. I noticed the new spring has some kind of coating on it. I did it when I did the water pump. The spring keeps the hose from collapsing once the water pump really starts moving coolant as the RPM's climb.
It could also be that there is a boatload of crap between the rad and the a/c condenser, that used to happen to my 2.8L S-10 every year, I'd have to unbolt the rad and tilt it back, stick a shop vac in there and vac up all the bugs, leaves, feathers, etc that got stuck in there. It would run fine around town but once I got out on the hiway the speed would swirl the stuff up and paste it to the radiator face, then when I'd stop it would all drop off after a few minutes and cool down.
Now my 98 runs at below 210 [guestimate 200], it used to run at exactly 210 even with the pump leaking, now it only climbs to 210 when heading up long hills in 3rd. Just some ideas to look at..

First off do a search for cooling problems. There has been a lot about this in the past month. Do what RichP said, look for debris in and around the radiator. Use high pressure water to flush all of that area. My bet though you have a plugged or semi plugged radiator. I had the exact problems about a month ago. My radiator was 75% plugged. I had the radiator rodded, and put in a 185F thermostat. It now does not get to the 210F mark anymore.
Good Luck,
SeanM.