Overheating

[thornburg]

Well, I never really solved my earlier problem (it pretty much mysteriously fixed itself... maybe the suggestion that it was "a bad tank of gas" was right), but now I have a new, much more concerning problem.

On the way home from work today, I noticed that the temp guage was rising a little bit in slow traffic, then it went back down. Then it went back up, and it got high enough (about 85%, my gauge has no degree readings) that I decided I had better turn the heater on to cool it off. Running the heater it returned to the normal reading (50%) with minor fluctuations for the rest of the ride home.

Upon arriving home, I checked out under the hood (not that I really know what I'm looking at), and pulled out the Haynes manual. The fluid level is good, and there does not seem to be any coolant leak. The fans work (at least some of the time--I suppose they could be intermittent), and, based on the test in the Haynes manual, the thermostat is at least partially working. I'm wondering if something could be clogged inside the radiator or coolant hoses, reducing (but not quite stopping) coolant flow, or if it is possible for a thermostat to get stuck half-way (i.e. not fully open and not fully closed).

Any other suggestions? I'm totally new to this "fix your own car" stuff, and I can't afford to shotgun the problem with parts that may (or may not) fix the problem. I also can't afford to take it to a shop (although if I really have to, it will just have to go on credit, and then I can have them work on my extremely intermittent rear drum lockup).

Details: 1990 Toyota Celica ST, 1.6L 4AFE engine (shared w/Toyota Corolla and Geo/Chevy Prizm), automatic. The radiator is only about 6 mos old (not sure exactly, but it was replaced shortly before I bought the car, and the radiator looked new at the time). The hoses don't look new, but, as I said, there don't seem to be any leaks.

Sorry for the long post. Any and all help is appreciated!