Considering you took it to a shop, and said that they could find nothing wrong, and barring the possibility that the shop employees are complete idiots, then I don't see how they could have not-diagnosed or mis-diagnosed a common head gasket problem... Besides, you would have noticed the excessive amount of thick white smoke, if you had blown a head gasket... if anyone has ever seen one blow, you know what I am talking about lol...

When was the last time your water pump was replaced? Did they replace it at the recommended interval? it's got 173k miles, and it's a 91, so I figure it should have had at least 2 timing belt changes, along with the recommended water pump replacement. It could be the water pump...

I had my 93 dx for 2 years before I got my swap, back in '00. The timing belt and water pump were replaced at the required interval of ~60k miles. I did not replace it again at ~120k miles because I was waiting to get my swap. Around 125k miles, I experienced the same problem as you describe... Temp would rise, and the only way I could get it to go back down, was to rev the engine up... I assumed this was because the water pump had gone bad, and it was not flowing enough coolant through the motor, until I would rev it up, to get the coolant flowing... Never figured it out, like I said, we tore out the D15 and put in a B16A2... but I always suspected it was the water pump...

it wasn't a head gasket prob, as it didn't burn any coolant...

Hope this helps!