I have a 1992 Lexus SC400 that runs flawlessly except for one problem----The idle speed does not stay constant. When cold, the engine will not start unless I feather the gas pedal, keep it at 1000 rpm for 30-45 seconds and then slowly back off the gas until it idles at around 450 RPM. After the engine warms up, it increases to about 700 RPM and all appears to be normal until I drive for a short period and then the idle speed goes up to 1200 rpm and cannot be kicked down. I have checked for sticking linkage, lubricated all moving parts on the throttle body, adjusted the idle speed screw, removed the IAC and completely cleaned it, and added gas treatment to premium only gasoline. What is the problem? ECU?