Either your thermostat is not an OEM t-stat, is bad, or your head gaskets are leaking internally.
Your heater core being bad or clogged is also a possibility, but not a common occurence.
I'd recommend getting a hydrocarbon test done as soon as possible. It will tell you if exhaust gases are getting into your coolant. Is your overflow tank bubbling? Milky? White smoke coming out of your exhaust?
Check your bottom radiator hose for heat, not the top. The bottom hose is directly connected to your t-stat.