problem: since its gotten cold here lately (below 50 degrees) my engine takes a long time to warm up AND after its been driven, it sometimes goes cold (temp gauge drops all the way to COLD). this happpens if i coast down a slight hill, or if i simply keep my foot off the accelerator while coasting for a minute or so. im almost ready to put tape over my radiator. ive checked my radiator coolant level and it is full. the coolant reservoir level is near full too. i havent had my coolant changed or my system flushed for a few years but i figure its fine because the engine never overheated when it was 100+ degrees out in heavy traffic, so what gives???? any suggestions???