no heat when stopped?

i have a 1995 civic ex coupe with a 1995 integra b18b1 engine, civic radiator and fan. my heat never really gets hot in my car, always blows kinda warm but sometimes on occasion will get hot. latly when i come to stops or slow down i dont have any heat at all, with the temps being in the negatives latly it gets cold quick. Im gussing i either need a thermastat or heater core? what does it sound like to you guys I do have a whole integra dash with heater box and stuff here if it is the core i might as well swap that in but i dont want to until spring because that is a bit of work. my temp. guage always stays about the same and never reaches the half way mark - it more stays about 1/3 of the way and thats leads me to belive the thermistat is good. so any input?

thanks alot
Kyle

The thermostat could be stuck open, in which case the heat gauge would stay in the normal area. I would definately replace the old thermostat before going much further, because they are cheap, relatively easy to replace, and is most likely the problem.