P0300 / Engine RPM speed @ idle

Jordo!

You almost definitely have a vacuum leak somewhere after the MAF sensors or somewhere before (or just past) the primary O2 sensors.

It's causing the car to run lean, and leads to a lean miss (and probably high positive fuel trims).

You can try spraying carb cleaner around the intake area and near the TB's while the car is running to see if the idle changes -- when it does, you are closing in on the leaky spot. Failing that, you'll have to get it on a lift and check for evidence of leaks around the primary O2's. That will be trickier to pinpoint (might see black exhaust residue), so hopefully the leak is before the headers.

Good luck.

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