2k OB Ltd Cylinder Misfire Issue

Well, possible cylinder misfire issue. 200 Outback Limited, 2nd owner, currently has 170k. 1st owner kept up with maintenance very well, we have put about 25k since we bought it. I began noticing a misfire a few months ago. Seemed like it happened mostly at highway speeds when trying to accelerate, say for a pass or after deceleration due to random idiot conditions. CEL was on, was getting a random misfire code so I replaced the plugs, wires and valve cover & plug hole gaskets as they were leaking oil into the plug holes. I used aftermarket wires from the local parts shop. Problem almost went away, definitely improved, but over a few weeks it came back. I just replaced the plug wires again with Subaru parts and again the situation improved but didn't completely go away. However now it seems like it happens at lower speeds/RPMs rather than highway speeds. How robust is the coil in this car? It is the next part on the "Misfire diagnostic" list. Just looking for some feedback on potential/known alternate problems I might be facing.
Thanks