Thinking logically about this, you say the temp stays steady if you dont drive the car hard but then, if you open it up, the temp shoots up? If you then slow back down and take it easy then the temp drops back to normal. If that is the case, then i'd say one of two things is the probem. There is a head gasket problem, but you'd notice this elsewhere ie in the expansion bottle, loss of water etc, or, there is a restriction to flow.(system not up to the task) You say the thermostat works, but i have seen a similar thing before. Firstly prove the radiator core isn't blocked (probably ony do this by using a different radiator) but then, and I know this is a pain, you might find it worthwhile looking at the hoses. What I found once was a coolant hose internal had failed and one of the rubber plys had come loose inside, and this restrcited the piping diameter. When removed, the rubber fell back into place and it was only by chance I noticed a loose flap of rubber inside a piece of hose. The pipe was being blocked approx 2/3 when water flowed through it. When there was no major cooling requirement there was enough flow to keep everything cool, but as soon as the engine was being used hard, there was insufficient flow. I've seen people fit engine components with blanked off gaskets, so no oil/coolant etc flows at all, and also bits of rag, jointing material in places where it shouldn't be. I once was working inside a marine diesel engine (Pistons 900mm bore, stroke 2300mm) and in the crankcase I found a full size beach towel!! It had to have been there for at least 3 months! 
Do keep us informed of what you find. Always useful info.
Do keep us informed of what you find. Always useful info.
