My 2005 impala had the same problem. Here's the answer. The heater core is plugging up. When you sit idling, the fluid movement is slow, therefore the junk settles in the heater core, still allowing enough fluid to get through to heat. When you drive it, the fluid movement is much faster, therefore sending the junk into the heater core, plugging it up. To check for this problem: Get engine temp up to normal, feel both heater hoses. If one is hot and the other one is cooler, fluid is not passing through. Take off both heater hoses at the top of the engine, blow out fluid with low pressure.(5 or 10 lbs.) Fill both hoses with the right radiator flush. (Some engines are aluminum) Let stand for 4 hours. Flush with new antifreeze. (I used a garden sprayer) Stick the sprayer end in the heater hose and tape tight. My vent temp WAS 100 degrees. After fix was 152!!

heater blower stopped blowin.cheked all fuses-ok tested blower motor -ok changed switch module -no blower!All lights alluminate and trapper doors function properly.Tested for power(removed fuse to test fuse prong to ground) at 25 amp fuse for blower on Passenger side fuse box and no pwer at all it is dead .

Help if you can it is getting cold here in Canada!!!

Update from Nov 22, 2009: After many lost hours of searching found a bad conection in the dash forward of the control module for the blower switch.1.Remove the dash cover then2. remove the control module .you will see a wire harness forward of the module location.the big RED wire supplies power to the blower.Mine had a poor connection at the connector!Wiggle it around with the ignition key at the on position and the blower dial switch to the right.if the blower comes on it has a bad connection like mine.To repair it --3.remove the ashtray assembly -4screws and another little haness for the cigarette lighter then pull the blower harness off it's grommet then pull it down and separate the connector off and if it is burned up inside at the red wire you will have to dissconect the battery positive and cut the red wire at both ends of the connector and splice in a bypass wire.make sure to splice with 25 amp wire and wire swage splice connectors.Your auto parts place can sell you what you need for this.I bought a spool of wire and a bunch of splice connectors for 10 bucks.After the job before reassembling the dash test the blower on high position for 20 minutes and feel the wire at both ends of your splice repairs.If it really heats up alot your splices are no good and should be checked again.worked for me!