Q: There is something amiss with the performance of my car, but no warnings saying what.

I'm always driving my car, so I know when something's wrong. When I start it up it idles really rough. I've replaced a ton of stuff already. My temperature gauge is running low. My heater blows out cold semi warm air. I'm constantly sneezing inside of my car. I have replaced all the filters. I have replace the blower motor. I have replaced the idler air valve thing. I have replaced the antifreeze coolant reservoir. I have replace the idler pulley, tensioner belt pulley, and serpentine belt. I have flushed the coolant out of my car twice. But my coolant is a brown looking color. I replaced a sensor that controls the lights. Recently had an oil change done. I live in my car so I travel everywhere. I have lived in some cold areas. And I start my car at night to warm up, about 3x a night. It runs really rough. It feels like something is dragging. I've had a full tune up as well.

My car has 159529 miles.
My car has an automatic transmission.