Random Misfire at Idle

Hello, new to the forum but I'm having an issue with my gen2 that I bought not long ago and it's been driving me crazy. Started by throwing a misfire code randomly every day or two and since I've started repairs and fixed the fuel trims it became more obvious it was at idle, can't go more than a few minutes now without a CEL.

Year/Make/Model: 2010 Mazdaspeed 3
Mileage: 207000km (128000 miles)
Concern: Engine will lightly misfire only at idle, briefly bogs down to about 500rpm, light vibrations when this happens but not super noticeable. Builds boost perfectly and absolutely no issues while driving or flat out.
DTC's: P0300
Modifications: Stock, used to have a CAI
ECU/Tuning Software: Not tuned
Is the concern intermittent? Yes
Can you duplicate the concern? Yes
Recent Repairs::
- Found and repaired a large vacuum leak (returned fuel trims to normal from almost +12 STFT)
- Smoke test to confirm
- Replaced plugs with NGK Iridium pre gapped to ~0.028 (my mechanic also tried a set of his own plugs recently to be sure)
- Replaced the MAF and swapped the cold air intake for a stock one
- Full oil and fluid change
- Cleaned fuel injectors

As I said, driving me nuts and I have basically given up on guessing at this point. Any help would be amazing, cheers!

EDIT: Engine died today, seems like it was a valve seal slowly failing. Need a rebuild now

 

Welcome to MSO,

At 207k, a full rebuild would be good if you are planning on building. If you are more interested in a daily with some punch, consider a head rebuild. The bottom end is stout enough for stock usage as long as the tune/fueling and maintenance is upkept.

If you just want back on the road, Id suggest a head rebuild, but also include upgraded HPFP internals and a good tune from Freektune, Purple Drank or Stratified tuning (if you want to deal in Cdn funds). Dont forget to verify the VVT is good, now is the time to do it aswell.

Dont forget to post an introduction in new members. We might get to know you more and your desired direction with the car.