I was driving to work in my 89' Lebaron 2.5 Turbo all normal. When I got to a stoplight, I noticed a rough idle sound, it goes away when i accelerate but also the temperature gauge needle tried going higher than its middle point but the fan wouldnt let it. I get to my parking spot to check her out and noticed the radiator had half coolant inside, the coolant in the reservoir was boiling a bit. The head gasket is fairly new, previous owner had it done. Theres no white smoke, no coolant in oil, no coolant dripping near anywhere in the engine. In my lunch break, I did a test and had the temperature rise up to the " boiling point " and noticed the needle would go up in the middle then come down, only for it to come right back up after 20 seconds. Could this be a thermostat? also and FYI, my radiator cap is not spring loaded and the radiator still is about a year old. Thanks!