Re: Flushing radiator + Changing thermostat
It has been suggested here that coolant does actually make your car run *slightly* hotter - something to do with the heat transfer ability being slightly inferior to water. But it does give you a higher boiling point, and anti-corrosive benefits etc.
I would be thinking that possibly the radiator is partially blocked, and you might need a professional flush / clean of the radiator, or possibly even a new one. A radiator place should be able to pop off the ends of the radiator for an inspection as well. ( I don't think its technically to difficult to do yourself, just annoying and time consuming ).