Plenty of water in radiator. Heater works. We had a cold spell a couple of weeks ago and that is when I first noticed something wasn't right about it. During the cold spell the guage would come up to normal and then a few minutes later it would go back down to zero. I first thought maybe it was so cold out that it was keeping the engine cool but now we are in our 60's and it doesn't rise at all now. Any suggestions on what might be causing it?