My Gas HVAC is not coming on by itself. When I turn up the thermostat is comes on for about 5-10 minutes then shuts off before reaching the set temperature. It won't come back on until I flip the breaker switch. I also have smelled gas fumes at times coming thruough my heating ducts. What do you think the problem is?

If you are smelling gas,, I think you need to call someone,, very dangerous

Yep, possible carbon monoxide fumes. Possible cracked heat exchanger.