Tach Problems?

[f5fstop]

Check your connector at the instrument cluster. Probably not a sensor, since the tech signal to the cluster is supplied by the PCM (via class 2), and if a sensor was going bad, the PCM would set a code and illuminate the service engine soon light.
Could be the cluster going bad, or maybe someone has experienced this and knows the fix. But looking at the schematics, this is what it appears to be with my limited knowledge.