Rough idle problem - vibrates the plastic dashboard on my 2003 infiniti FX35 with Nissan VQ35DE 6 cylinder engine. 220,000 miles. Changed engine mounts, replaced intake/plenum gasket, ran Lucas additive through the tank, cleaned and adjusted throttle body and set idle at 650 RPM (holds steady). Replaced plugs. Still idles roughly. Any thoughts?

No. 1-1

those are notorious for blown head gaskets at that age. Do this video test first: and if not that, realize the coils often go bad on those