I recently replaced the thermostat in my car ?

The car was overheating and I blew a gasket. So I replace the head gasket and put it all back together. It ran good for a few weeks but now the car is running too cold. I put in 2 aftermarket stats and finally gave up with those and got an original part from the Honda dealer. It seems to warm up to the right temperature Then it drops again then goes back up. It never stays put. And if I turn on the heater it will drop almost to cold and not go back up to where its supposed to. I took it in to get it checked and the mechanic Said the temp Is where it's supposed to be at he put a digital thermometer and Showed me and it was between 185 and 195. So I turned on the heater an temp dropped to 160 to 165 is that normal. It never did this before even when turning on the heater it always stayed at the right spot on the gauge. For a while I was getting really bad gas mileage. Close to 12 mpg less then usual. Could it be the coolant temp sensor telling the car to run rich because it thinks it's cold. Or is 165 too cold to be running the engine at. My car is a 99 Honda civic ex d16y8 motor. I have tried everything an nothing seems to work. Please. If you could help me out. My car last got 31 mpg so it's back to getting good gas mileage as of right now but sensor still says cold. The original thermostat from Honda which I just bought is 172. Before that I had a 180 stat. Should I try the 195 stat and see if that works. I would think that the original one from Honda would work no problem so It leads me to nothing. Please help thank you