In May 2012 I had the thermostat changed to one that can handle hot, high desert conditions because my engine was running hot. The mechanics in Arizona replaced the thermostat, a cracked radiator cap, flushed the radiator and added a "special" radiator fluid. It diffused the problem, although it ran hot when climbing hills and gaining elevation (mountains and passes). They also added this purple stuff into the radiator and checked for a blown head gasket, and found it was fine.
Now, in January in Northern California, the engine is almost immediately running hot - at the beginning of the red line. The temperature gauge is fluctuating all over the place and randomly. I live in an agriculturally flat area. When the radiator starts running hot I kick on the defrost full blast and the highest heat. Some times it helps, other times not. During stop starts I hear a swish and gurgle of water flush through under my dash. I haven't heard that sound lately. It is getting worse. Last night it blew cold air for the first time. Today it ran in the red. It has never steamed over or blew the cap off.
I'm beginning to think the erratic temperatures are partially symptomatic of my high altitude / hot climate SW thermostat not liking the cold low altitude. But now there's the new symptom of blowing cold air, which makes me think it's the heater core.
My budget is not allowing me to take my truck into a shop, so your insights could help me to help myself fix my truck, which I need to get to work. Thank you for reading.