Misfire codes, among other issues

[Jake17gol]

About a month ago, I purchased a 2017 GT CPO from Dallas. About a week and a half in I was already having issues, but the worst of them all was the misfires. Codes are P0316 and P0308 which is misfire at startup and misfire on cylinder 8. The first trip to the dealer, they swapped the plugs and coils for 6 and 8 and tightened the air intake or something like that because it was loose. Misfire continued. Went again to a different dealer, they swapped the intake manifold and gasket, the CEL is no longer there, but the codes are still up when you check with an OBDII scanner. Could it still be fixed and the codes just be residue? Do I have to just wait for those codes to go away themselves assuming it’s been fixed, or is it still misfiring since the codes are still up? Appreciate any input. On a side note, my car is completely stock aside from an axle back, but even before that exhaust install, a squeak/creak developer in the rear. At first I thought it was as the rear suspension, but now I’m not so sure. What could it be? Another issue is the bump steer, it’s awful. I don’t feel like it was this bad when I got it, but I’m not too sure. Do mustangs just have bad bump-steer from the factory or could this be another issue? I drive around 7 different cars on a regular basis and none of them have bump steer like this mustang, not even close, except for the ford transit van which makes me think it’s a ford thing. Although the van is still not as bad as the mustang. The mustangs steering also seems to “stick” sometimes. For example, it’ll out of nowhere turn by itself and stick a bit to the left or right. It takes some force to get it back straight. I read this is just due to fords awful implementation of electronic steering, but I’m not sure, seems very unsafe especially on the highway. I’ve done an alignment so that’s not the issue. I’ve heard it could be the struts. The last issue is this weird sound I can’t describe, almost like a sizzling or boiling sound coming from the rear when driving on the highway at 60mph+. What could it be?

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