It can even be something as simple as the screws holding the cluster in loosening up over time. Try pushing the cluster in a bit and see if the reading varies. If it does, try tightening the screws a bit, but don't torque down on them. You will strip the plastic. Use a nut driver to keep the torque down. If this does not solve it, take out the cluster and clean the contacts. The bolt heads should be 9/32 or 7mm. If they are a little smaller, they are 1/4 in GM used both sizes on the Cutlass cluster and I assume they kept that convention on at least some of the other models. (I was just working on mine the other night)
