My 92 Honda Prelude is overheating and no one knows why! Can you think of a reason/theory and explain how to test these theories?

My car overheats when I drive it for 45+ minutes on the freeway. I'm usually going anywhere from 45-80mph. I don't really know how it would function on local drives since if I'm driving far, I wouldn't take surface streets.

I checked the radiator and it's not leaking. I took it to a mechanic and he can't find anything wrong. They used a pressure tester and said it's not leaking.

So here's a little story.

I'm driving for a long time on the freeway, and I notice that the temperature goes up. I freak out and slow down to take the next exit. As I slow down, the temperature eventually drops back to normal range. So I get back on the highway. As I speed up again, sure enough after a few minutes the temperature goes back up... so this time, I blast the heater and the temperature goes back to normal and I just finish my drive this way.

My long drive is to work by the way.

Anyways, I get home and the next morning, check if the radiator needs filling... nope, all full. Not even one drop needed. So I continue to drive this way for a few more days and then on the 3rd or so day, with the heater on, on the way home, the temperature goes up. I slow down and it goes back down. I yo-yo the temperature for a bit, but get annoyed with driving so slow so I pull over to a gas station. I spend maybe 15 minutes waiting for the engine to cool. It was still pretty hot but not blazing hot when I opened the radiator cap to check if it needed a refill... and it did! It guzzled maybe about half a gallon of water. Once it was refilled, I drove back home without a problem (with the heater on of course).

Can anyone figure out this mystery? I just took it to the mechanic again with this problem and they're guessing it might be a faulty cap. I don't need to go to work yet so I can't test this, but I don't think it's the cap. I would think that if it was the cap, I would see steam coming from it.

Please help!

Also, on a possible related issue, but probably not, my car doesn't idle well. The RPMs go up and down when idle and at a complete stop. It's pretty sporadic on when this happens though. Sometimes, driving around will fix it, sometimes it wont. After a long drive, it might do it for a while after I get off the freeway but after driving surface streets for a while it might go away... so keep doing it. It's really random.