2004 Chevy Colorado RPMs drop when vehicle comes to a stop

How many miles? Replacing the oxygen sensor on my roommate's 1990 Honda Civic cured the same problem, which he had whenever the A/C was on (car has 219,000 miles). Bad oxygen sensor makes for bad fuel mixture. Makes it more difficult for engine to idle properly. Engine must make use of secondary back-up devices to make up for the problem. Your problem could also be due to a faulty throttle bypass valve (if your car has one). Could be due to a faulty throttle position sensor (or the device that controls it). Could also be due to a lot of other things, such as the computer, etc, though the items I just mentioned are the most likely causes, I believe.