Tore apart center console today, checked all cables from the ECM there, sanded down ground connector, used a stainless steel screw.
Went at the C500 connector again, replaced positive cable from battery to underneath C500 block, replaced negative cable to engine, replaced ground cable from engine to chassis, sanded all surfaces.
No change, idle still hunting. Scanner / diagnostic kit won't arrive until sometime next week. Had fun troubleshooting in the beginning, sort of a challenge.
Now frustration is setting in.
\D