Another hunting idle

Howdy, I'm new to this forum and STi's in general so be gentle....

I am currently testing a 99 JDM STi engine on a test rig in preparation for transplanting into a Porsche Speedster replica (yes really!). It starts, runs and accelerates OK but once it warms up it is hunting at idle.

If I temporarily switch of the fuel pump (an external Fuelmiser FPE-240) then it idles normally but as soon as I switch the pump back on it starts hunting again. This suggests to me the pressure has something to do with it so could it be the fuel pressure regulator is shot, or could the pump pressure be too high?

I understand the Furemiser pump specs are 70psi and 250 litres /hr. I couldn't find anything on the Interwebs telling me what the equivalent specs are for the standard in-tank STi pumps, can anyone enlighten me?

Thanks.