I really couldn't say - I am an expert in nothing I'm sorry to say. But it does sound plausible.
The TCO may be operating correctly - in which case the water would get hot and then very hot for a few moments and then the TCO would open and the water then run cold.
If the TCO were permanently tripped (open circuit) then the water would never get hot.
I guess that these TCO things are self-resetting which is to say that after an overheat situation they cool down and allow things to go wrong again - but I just guess.
If it were my shower I would short circuit the TCO as a temporary test - just for a little while - a few seconds but I'm oblivious to danger.
The question is - is your TCO working correctly? Is the TCO at fault or is there another problem?
Do not look to me for a solution - I am without knowledge or experience.
Good luck...
Peccavi
July 2013