1998 Honda Civic Overheating - can't figure it out

So there are a few factors that I think may contribute to my 1998 Honda Civic EX and the overheating problems that I am having. Here is a detailed history of the problem from when it first began to occur:

1) About a month ago, the water pump went out on my car. It began to overheat, even on cold days. Also, the heat in the car stopped working around the same time that the water pump broke. After one or two times driving the car (and it overheated within 5 mins) I realized it couldn't be driven anymore. The timing belt and water pump and gaskets and seals were all replaced by a certified mechanic. The car seemed to run perfectly fine around town.

2) About a week ago I was making a 2 hour drive at highway speeds (65-75 mph) and noticed that the temperature gauge began to fluctuate between normal and hot. As it was on "H", I pulled over and opened the hood to find that the overrun reservoir had boiling coolant, but no leaks just a bit of steam. Without accelerating or idling, the gauge continued to fluctuate. The heat in the car also continued to not put out any heat (or at least very little warmth). I finished the rest of the trip without a serious overheat, but the heat gauge constantly fluctuated from normal to hot temperatures moving up and down quite rapidly.

3) I replaced the thermostat. Still not blowing any hot air, I have no heat. The gauge continues to do the same thing. The fans on the radiator are turning, the water pump and thermostat are brand new. The head gasket is not damaged (as there is no oil in the coolant or visa-versa).


I was told it could be my heating core, but that would not explain the problem of overheating. It may explain why I have no heat, but I've been told that the overheating is being caused by something else. I am absolutely stumped, and I don't have the money to experiment and keep buying parts that aren't going to fix my car. Any ideas what the problem might be?