It sounds to me like the lambda system is working. I would think that in that range the idle would be OK. You could have a failing idle air valve or vacuum leaks. Agree - change plugs back to copper.
You need a wiring diagram to figure out what the coolant sensor does. On the 380, the oil temp kicks the lambda on and the coolant sensor controls the idle.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe