Test the radiator cap to make sure it's holding pressure, pressure test the cooling system. It's not uncommon to have a coolant leak which only leaks at speed and doesn't leave pools behind after you stop the car. Yes, I saw that you lifted the car... I've never had to go through the elaborate bleeding techinques recommended by so many owners and I've worked on a lot of MKIIIs. Fill the coolant system, run the engine with the heat on for a minute, top off the coolant in the radiator and then just keep the reservoir completely full for a couple days. Works for me every time unless there's a leak which has been missed or the HG isn't sealed and compression gasses are entering the system and pushing coolant out of the coolant circuit. It's not something Toyota just assumes all techs know either, you should look at the official procedure for the MR-2s. I'm told that clipping off the tiny brass stopper on the thermostat (just a little rod in the tiny hole in the flange of the thermostat) can expedite the process I outlined with no ill effect.
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