I would drain rad, flush system, (good time to replace all hoses if need) replace thermostat refill with anti-freeze. Start car with the red cap off and you standing there with more anti-freeze, when the water starts moving in the rad, rev engine up slightly (to keep water from gushing out), continue to fill rad. When you get it full put cap back on and let idle. If you are outside, you may want to put cardboard in front of rad to block air flow an warm up faster. Don't forget to remove when finished. Thermostats will stick open. Wont really show up till it gets cold outside. (when it's warm out you just don't notice it takes longer to warm up) At a idle or slow running, temp will not always reach full heat. Engine is just not building heat, you have your heater (which is just a small rad cooling antifreeze) cooling the water. You can put a covering over the rad to block airflow, good idea to put a sticky on dash so you do not forget about it and overheat it later. Sounds like your anti-freeze may not have been serviced lately, good starting point. If you have good heat an gauge is reading low, sending unit then gauge check.
