My speedometer does not always work it will sometimes read that i am going 120 mph when im going slow or stopped please help

Hello, this problem is due to a failing stepper motor. The stepper motor is the motor behind the gauge that controls the movement of the needle. When a stepper motor begins to go out it will cause the gauge needle to stick, skip around, stay at zero or max out on the gauge. The only way to fix the problem is to replace the stepper motor with a new one. While replacing one motor it is best to go ahead and replace them all as the others will go out in time as well. Your cluster will have one motor for each gauge. Here's a link to a page showing exactly how to replace a GM stepper motor and where to get the correct parts to do the job! DIY Finally How To Repair GM Instrument Cluster At Home Speedometer And...

Faulty instrument cluster