temp guage kinda normal, but no heat sometimes.

Ok i am at the point where I want to just get rid of my truck. I've had this same dam problem for a year now and no fix has been helpfull at all. It all started when I had those 10 min. oil change places flush my system. A few weeks later I noticed that the temp guage would never rise properly then my truck would over heat. If I turned the motor off, i'de hear a pop sometimes and the coolant would go down and I'de have heat. Mechanic flushed the system again, replaced tstat, cleaned out radiator to no avail. Took it to ford, cause mechanic said it could be headgasket. Ford replaces the waterpump, kinda helped but still had problem, just not as often. Then the spring and summer come, NO PROBLEMS AT ALL. IT ONLY HAPPENS WHEN ITS COLD OUT! Untill, the first cold day 2 weeks ago. Driving to work, hit every red light, temp never would go up. Give it gas, go up. hit red light, temp go down. Eventually it would goto normal and I'de have heat. Then on last thursday, im reving the engine from the tb and notice the resevoir starts to rise. To the point of overflow. I shut off the engine and hear the coolant rush back. I start the truck and drive to my mechanic. I tell him whats going on again. At first he and another guy said it could be the blend door actuator since sometimes the temp would be normal but I'de have no heat. But then when I leave it overheats, take it back. Says it has to be the tstat again.
Now, monday have it flushed, new tstat, synthetic ford coolant added. Seemed to help, heat seemed to be hotter, no problems till yesterday night. Temp took longer to warm up, and had barely any heat. Tonight, no problems after work. Nothing till i decided to go out. Drive around for a bit, temp was not at cold after sitting. Started to warm up, then i started to loose my heat but not to the point of cold. Kinda cool, had to keep it revved at around 1500rpms to get it hot again. The whole time the temp guage is at normal. This happened twice. The second time it kinda was luke warm. The heat very gradually came back and stayed. I've run out of idea's, options and money.

What the hell is wrong with my truck?

What are we working on Steve? Things to check re-gardless, Vacume lines, does your truck have a Hot water shut off valve?, if so check the cable ... check the vacume line to the vent area near the heater, you may not be switching over from cold air to warm....

What I would do is check to see it the bypass is working. Then take out the thermos(only in
cold weather) and put a brass washer inplace so that the middle is reduced to about .75"
hole. Run it around the area to see if it heats up; holds it's own or runs cold. An old trick of
elimination.

well whatever it was is gone. I drilled a small hole in the tstat and added a bottle of water wetter and it went away. It got me thinking that it was electrolysis.