Why does my car's(2000 Honda CRV) temperature gauge sort of fluxuate in cold weather?

My car will heat up fine if I let it idle for 10-20 minutes, but when I start to drive and put the heat on it immediately starts to drop and fluxuate. I live in a town and the highest speed limit I usually go is about 45-50 on a state route road. I do get heat coming out but it doesn't seem to be as hot as it should.

I checked my coolant and it was low, but even after filling it it still fluxuates but now it seems to at least stay over the cold line. My first thinking is the thermostat or maybe a clog in the radiator hoses but I'm not entirely sure and haven't had time to actually check anything out.

Maybe the thermostat is opening and the water is circulating more. Try changing that part and make sure your antifreeze to water ratio is correct.

Faulty thermostat.

It's not closing properly and so allows too much coolant through the radiator in the cold weather. When you are stationary the radiator doesn't get rid of the heat, so the engine warms up to normal.

When you are driving you get extra cold air though the radiator and this cools the engine TOO well. Normally the thermostat would close and keep the engine temp perfect, but that's not what happens.