Here is what I would try and why.

Because you're not overheating during auto-x driving, but you are overheating during in-town day to day driving. I would also asume you're not overheating during freeway driving?

If this is the case, my best guess would be that your fan sucks. Either its old and slow, or it's dead, or something. So here's how to check if it's working at all. Drive till you start to get too hot, and then pull over, leave the engine running and pop the hood, see if the fan is running. If it's not, then it's dead and you FOR SURE need a new one. Even if it is running, it may be running really slow and you might think about replacing it.

The reason I think it's the fan, is...that when you're driving, the wind from you driving your car is creating enough moving air to cool the water in the radiator, and therefore cool down the engine. But when you are driving in town, you stop at red-lights ect, and when you do that, the fan has to kick in and create wind to cool the water in the radiator. If it's dead, or slow, it won't cool the water enough and then the engine gets hot. I don't know if you have a new fan, or mounted the old one to your new radiator, but if the old one is now mounted perhaps it's mounted in a funky way? and is now binding on something? Just an idea...

as for the "special fluid" I've never heard of running special fluid in aftermarket radiators...call the people you bought it from and ask them. But I would bet the guy who told you that didn't know brake fluid from transmission fluid and doesn't have a clue.

Good Luck! Be sure to post up what finally fixes it! :-)