Check the simple stuff first!
Radiator cap rubber seal should have NO visible cracks.
Symptom of temperature correct when continuously driving, trending higher in stop and go traffic suggests thermstat.
With only 71K miles I suspect that the thermostat has never been replaced.
That said I also suspect that the coolant may never have been replaced.
If/when replacing the thermostat do a full cooling system flush, that will give you much better idea of the condition of the entire cooling system.
Sulfur smell when the AC is turned off; check to see if there is a small coolant leak in the heater box. If so, this is a precursor of serious problem with the cooling system.
Last edited by gen1; 04-06-2017 at 05:12 PM.
Reason: added suggestion