Engine revs high - overheats in hot weather

2000 Honda Civic SE (DX) // 120,000 miles

I have a unique problem, when the mechanic finally understood he remarked 'oh, that's weird.'

When the weather gets warmer and the temperature is 75+ degF, my engine starts to rev really high after I've been driving for a short time. This is not a problem when I'm at high speeds, but stuck in traffic at slow and stop/go speeds this becomes serious because the engine keeps revving way too high. I don't have a tachometer but I know the car is close to redlining and will blow a gasket. I had white smoke once (2 summers ago), I still might have a leak in small amounts, BUT

- I've never had to top off my coolant. I've had this problem a few years, I just avoid using the car in hot weather + traffic AND

- when the car is revving out of control, the thermostat gauge stays in the middle, it doesn't rise. I had the thermostat checked today, it's working fine.

+ the ECM board isn't showing any errors

I was told that there could be a small crack in the head gasket causing this, and it will get worse, but I've had the problem at least 3 years. Mechanics can't seem to solve it so far. Help!