I'll only tell you this because it happened to me. I had a 82 Celica, in about 1990 I replaced the water pump, all was well. From about 92 to 96 the temps in the summer increased a little each year, by 1996 on a hot day it would go into the red. I checked everything I could think of, everything was fine. Thinking back to the water pump replacement, I remembered it had an unusually thick gasket, and wondered if the flow was compromised due to the impeller being too far away from from it's mating surface in the block which would cause churning I surmised. I removed the pump and gasket and bolted it back up with just RTV as a gasket. After that the temps went back to dead center on the gauge and never had another heating problem. The impeller was just spaced out too far, why it increased in temp year over year I do not know, there did not seem to be any corrosion on the impeller or corresponding surfaces, nor did it appear that the impeller was moving on the shaft.
I am not saying this is your problem, but it was mine and may be something to think about.
