Honda - Accord :: 1994 - Overheating When Hit The Highway And Accelerate

Alright, car's overheating, but generally not at under ~55mph or so. When I hit the highway and accelerate, the thermostat climbs and hovers 3/4s of the way to red. But only sometimes. Sometimes, often when I'm going down a hill, it will swings down to normal. Once it's gotten hot, if I take it off the highway and am idling or driving in the 15-40mph range, the temp gauge will spike. But only sometimes. Sometimes I'll hear the fans kick in while it's idling/overheating. The temperature will go back to normal, and then rise again and stay there or spike (seemingly randomly). It's never reached all the way to the red, but it toes the line. And I take the darn thing into the shop and it refuses to overheat.

I've taken the radiator reservoir out and there seems to be green fluid (antifreeze/coolant?) in the water (have flushed it repeatedly)...and over the course of several weeks, the water level itself will go down. Both fans and the thermostat have been replaced in the last 6 months. Additionally, the timing belt and serpentine belt have been replaced within 5 months, as have affected pistons.((S/N--RPMs are really....low? ~2500 @ 75mph? but don't seem to correlate w/temperature. would they effect it?)

I took it into my shop and they said "we ran all the tests and couldn't find anything wrong." Cool? I'm at a loss..