. 1996 K1500 Suburban with a 5.7 Vortec. From a cold start start, the temp gauge will rise to just above the 210* mark, then fall to about 150* to 160* and stay at 160* until I get into town, then it drops to about 150*. The heater doesn't get very warm because of this. I've replaced the t-stat and it operates just as it did with the old t-stat. Where should I start?