This is a very common issue with the '05 Impala's. The stepper motor for the speedometer has a broken gear in it, causing unpredictable operation. If the odometer is still working, then the speed sensor is OK. Instrument cluster needs repaired or replaced.
Want to save some green and replace this component yourself? Just ask and I shall post instructions for you.
Hope this helps.
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Saturday, January 10th, 2009 AT 1:34 AM