In neutral, RPM gradually falls off to 500 or less, then zooms up to around 1000. With the car in drive the variation isn't as great but the engine is 'bumping' against the transmission, very noticeably. It goes away when the engine reaches normal operating temperature. My 'trusted mechanic' has basically given me the 'can't reproduce' (meaning no engine check light or OBD2 fault code and he doesn't want to try to troubleshoot without that, fail to find the cause, then have to get me to pay for his labor). But it's definitely there, anybody else had this?
Possibly relevant earlier problem: a couple of months ago I did get OBD2 code P1445 (secondary air injection system valve stuck closed) after engine check and VSC/off lights came on. But it went away by itself, first the warning lights disappeared but OBD2 tool still registered the fault, eventually it too showed all clear. But 'secondary air injection' is an emission system operating when engine is cold, now cold idle problem, related?