Just finished changing the intake manifold gasket. Cranked it up to see if it was any better. Still rumbling and stumbling, although maybe not quite as bad. Pulled a P0102 and P0306 out of pending codes. The P0102 went away on the next run. I cleaned the MAF while I had everything apart so hopefully the P0102 was just a remnant of that. However the P0306 was still there so I'll look into plug #6 once the engine cools down.