dubdup

Hi, New Subaru owner, just bought a 53 plate STI 3 days ago. Did all the usual checks before buying (started from cold etc). Anyway past few mornings the car starts fine, idles at 1600rpm. If I try to set off, just flat spots and now power. I actually stalled it day before yesterday - so thought it was me. It does not hunt or misfire, No stall yesterday, but same result no power. So cleaned MAF sensor, still same. So left it on tick over today, once the temp needle starts to move, off we go and it is perfect drive. Will rev freely when I am having this problem. Running V power fuel, no mods (except an K&N panel filter). Quizzed last owner and he said has experienced this, but has always let it tick over before and after driving, so never been an issue, also he had it on diagnostic at garage and he said they found nothing. My concern is, it's not normal and don't want to be stuck as weather gets colder. Anyone any ideas or had the same? Thanks from a Subaru Newbie