There's a harness ground on the back of the engine on the passenger side, a little 10mm bolt holds it on. It's buried under the intake, not the easy one right on top. I believe it needs to be taken off when replacing valve cover gaskets and if it gets left loose it'll cause a very similar problem. Probably a shot in the dark unless you recently had that job done...(requires upper plenum removal.) It's a very important ground for the ECM. You could let it idle and wiggle the harness back there with long pliers, see what happens. And again, it's buried under the intake
