So I have a 2017 GMC Sierra 1500 with the 5.3 at 35k miles and I got a flashing CEL for about 30 seconds a few weeks ago on the highway and took it it to auto parts store but it had no codes present. About a week and a half ago the CEL came on constant and it came up with the P50D Rough Idle and so I got oil changed and ran some fuel system cleaner through and after a couple days the light started flashing and ran rough for a few min, I parked it and started it 10 min later and it was a solid CEL again but I was able to pull a P300 General misfire code. Today it did the same thing but ran worse than ever and I thought it was going to die on me, shut it down for 10 min and it ran ok. Before I start throwing money at this where should I start, spark plugs, coil packs, injectors? I'm at a loss with only 35k and just passed the 3yr warranty. Thanks for any advice.