Buick - Lesabre :: 2005 Stalls While Idling At Stop Lights

Last November my car began stalling while idling at stop lights. It didn't happen all the time, just occasionally. The car would start back up without too much difficulty. I took it to a GM dealer who cleaned the throttle body and decarburized the intake and exhaust valves. They also removed an aftermarket security system which had been installed by the previous owner which they claimed may have interfered electrically with the engine. They reported a history of code P0128 (thermostat sticking open) being stored in the ECM. I did not have the thermostat replaced.

The work the dealer performed did not resolve the issue. During the winter, I've been letting the car warm-up for a few minutes before driving. While warming up, the rpms will sometimes increase or decrease 200-300 rpm then return to normal idling a few seconds later. Sometimes when the rpms dip down low enough, the engine sputters and either recovers or stalls. On most days I don't have any issues and the car runs fine. The check engine light sometimes comes on while this happens, and sometimes it doesn't.