Misfire under load

Been trying to diagnose a p0300 code that's accompanied with b1318. Right about ~10 pounds of boost I get random misfires and battery voltage reading on AP drops to 13.0-13.2V for a little then slowly raises back up. I've checked voltage drop across everything I can think of except for alternator positive to battery positive. I'm thinking it's either a bad alternator or serpentine belt, but wanted some second thoughts. I have also done a boost leak test and did find a leak where the intercooler attaches to the BPV, but stopping this leak didn't fix the issue.

2007 ms6

Thanks for any help.

EDIT: some additional info, I can clear the codes (p0300 and b1318) and have them not come back at all until I start boosting heavy again

EDIT #2: Some additional info. Boost leak test checked out fine (all air leaked out of the valve cover breather and no where else). I replaced the serpentine belt cause it was $30 and why not? Didn't fix it sadly. Still gotta pull the alternator and get it bench tested, and possibly try some diff spark plugs i have laying around.

Edit #3: I went in and changed the plugs, spark plug on cylinder #4 was loose. The ignition coil was a little sooted up tried my best to clean it with rags and MAF cleaner (figured i'd use something less abrasive than brake cleaner). Anyway, swapped the plugs and clean the coils and car runs fine now. I'm still at a loss why I would only notice this misfire under heavy boost, and the b1318 code has me confused as well. However, my car is running much better and I'm happy. Solved, and hope this teaches at least one person to CHECK THEIR DAMN SPARK PLUGS.