Replacing factory tach

Sounds like a bad stepper motor or the gears. I dont know of any fix....never heard of anyone actually messing the these clusters indepth. Most will just replace the unit if it has problems.

Do you have gauge overlays? Are you sure the needle is not getting caught up on something? Sometimes the gauge overlays can bubble or just not be seated completely flat and cause the needle(s) to catch on it. The needles for the gauges nearly float effortlessly, so it wouldn't take much to snag a needle.
I'd start by pulling the cluster and popping then reseating the needle. Best tool to remove the needle is a kitchen fork. Be gentle. Then reseat it (using the diagnostics to help you get the needle pointing properly). It takes about 20 mins to get the cluster out, 10 mins to fix and 15 to put it back in. It's certainly worth a try before replacing anything.