Seen the newer mazda 121s? http://www.google.co.uk/search?q=mazda+121+fiesta&client=safari&hl=en&tbm=isch&tbo=u&source=univ&sa=X&ei=05UkUv2RLcmphAfmiYGIAw&ved=0CC8QsAQ&biw=320&bih=416
Blockage? Sounds like some form of air lock or some pressurising although obviously you say not, sounds like something's pressurising it out of the rad and once engines off its allowing it back in.
Temperature staying the same?
Could be an air lock, too much water possibly, headgasket, cracked head. Check the usuals, pressurising coolant system, gunk under oil cap, petrol smell in coolant etc etc
That's a very nice turbo, td05h 16gs are commonly used on starlets with good results. Td04 is more of an entry turbo, people looking for more power go onto td05/vf series quiet often. I suspect that's some form of hybrid, I'd be inclined to message Ricky on here (racetech) and he'll probably be able to advise you on the turbo. I ran a td05 on a stock engine for a few months, you adapt your driving to compensate the lag. It was good fun!