update:-
its the MAF. I put the old one back in and the problem is still there, but much less so than before. looks like I need to source a replacement.
The part code on the MAF is 22680-AA160 (Green label).
Looking on ebay, they seem to be listed for the GF8 which is 218BHP. as mine is running with an STI ECU that's chipped and with a few other mods should be running between 300 and 320BHP. in view of that, would a stock MAF be ok still?
I have a funny feeling the green label was used on early WRX and STI's too, which if that is the case, then a new one should be fine for me.
I am looking at
this on ebay