I had a very similar sounding problem, after paying Ford $520 to tell me they couldn't find the problem....Me and some mates decidided to change the spark plugs as it seemed to be an electrical break down at 3500rpm. When removing the plugs we found that the third plug hole was filled with oil??? It seems that the seal for the rocker cover around the plug had failed. This was causing the spark to leak under load, hence the stutter. I removed all the excess oil from the hole (all I did was remove the plug and the oil drained into the cylinder) and replaced the plug. I then went down to my local Repco to get a gasket kit only to be told that only Ford sells them. I still haven't got round to replacing the gasket, it hasn't done it again. Ford tried to tell me it was the wastegate failing....?
