My 2004 impala base 3.4 engine has about 101K miles on it and a new IAC motor replaced since the old one for some odd reason, was idling fast(about 1100 RPM) after driving on freeway with A/C running when at a stop and in park. The throttle body is cleaned, been tuned up (new A/C Delco (iridium) plugs and wires), fuel pressure regulator replaced due to seepage, and fuel filter replaced. I replaced the IAC valve with a BWD brand. After I shut off the car and then restart it after a few minutes, the idle speed would go back to normal. It happened yesterday after driving on the freeway at close to 70 mph for about 5 miles distance. What can cause this intermittent problem?