have u tryed flushing the system an is ur rs turbo running rich cuz that can cause it overheating take it to a proper tuner eg jano oddkiddcreations or call him on 07984071567 he will guide u with what is happening hes a very helpfull guy.

cud be the following eg:

air lock in system

thermo

faulty guage

radiator

to flush the system easy way is to take the pipe off the top of the thermo wen running an let the water come out then put the pipe back on that shud do it if thats any help hpoe u find out what it is. when u find out what it is let me know what it was just post on hera so every one knows what it was thank you.