Random Misfire 3.5 Right Bank Only

My '06 3.5L SE with 156k has developed a problem - I'm getting an 0300 CEL code and when my mechanic runs a scan it comes up on the right bank only and on all three cylinders in that bank - randomly. Symptom is a slightly rough idle, but no appreciable degradation in performance. The hotter the engine gets, the worse the idle, but has not gotten undriveable. Intake was pulled and new plugs installed. Water pump and timing belt were changed prior to this issue cropping up. Two of the four O2 sensors have been changed (was initially getting 157/158 O2 sensor code also). Exhaust has been dropped at the manifold and car run to see if that would stop the misfire (thinking possible clogged cat) - no dice. Next step will be checking fuel pressure to see if the rail is losing its oomph on that side. Car will not pass state inspection with CEL on. Any ideas or experiences with this type of problem? Thanks for the read.