do a compression test to see if there ain't a dead hole. Started out of the clear blue? any parts changes/adjustments immediately preceding the onset? internally balanced? as no weights that coulda fallen off (a stretch but wanna cover everything that comes to mind). Yes check that firing order (esp 5/7). I have a similar story but it is tragic so I will pass. I would never suggest purchasing parts blindly (shotgunning) to try to solve an issue but would you have another ECU and coil you could sub in (one at a time) to elim them. keep us posted