My 05 suburban speedometer started sticking at different speeds and started showing lower speeds than I was traveling. Turning off the vehicle resets it most of the time. What is going on?

 

it is due to a faulty stepper motor in the instrument cluster. GM has extended the warranty on the cluster to 7 years/70,000 miles. If you are over in the mileage you will not get any help from GM on it. Take it to the dealer and get them to replace the entire cluster under the warranty program.