More progress on this now.
I recently took the car to an auto-elec who ran it under an engine analyser.
There were no fault codes and he mentioned that he was looking at the sensor output in real time and everything appeared "reasonable".
It was misfiring on two cyclinders and upon taking the spark plugs out they were heavily fouled despite being replaced recently (< 20k ago, and they are platinum plugs).
Upon replacing them the car was running like new but I was certain that it wasnt the plugs as i checked them (he did give me the plugs as proof and they are fouled, but I dont believe this caused the problem, in my opinion the problem caused the fouling).
Anyways after driving it for a couple of weeks i can feel the missing/surging coming back again, so i think my suspicions were correct.
Upon inspecting the old plugs they all have "corona stains" two of which were very pronounced (see case 4 over here for what I am talking about -
http://www.globaldenso.com/en/products/ ... index.html). It appears that this isnt really an issue but i will investigate further.
I am going to do some more research on what could be causing the problem but for now I am completely stumped again. I just checked the ignition lead resistances and all are well within subaru specs.
If anyone has experienced similar issues or may know what it is then I'm all ears
