Pearlyred

Hi all, looking for some guidance as I have only basic knowledge of car engines.

Our 2005 Impreza 1.5i (JDM EJ15?) was playing up a few weeks ago, struggling to start and surging, missing while running. 

I’ve replaced the plugs, leads, in-bonnet fuel filter and air filter. This has fixed most of the issue, but it still seems to be low on power/acceleration. Also, as it moves through the rev range, there’s a spot between 15k-18k where it feels rough and there’s some vibration. Starts perfectly though.

It had an oil change and new oil filter a couple of months ago.

Can anyone offer ideas of where to look next? Could I have installed something incorrectly when replacing the parts above? The shop didn’t have the platinum plugs I’d intended to get, but had iridium that are also a match for this model. 

when I replaced the plugs and leads, there was a small amount of oil around the tube seals. Could that be bad enough to cause these issues?

Hope someone can help