Only advice I have is to check your grounds. I assume you measured a consistent RPM with the Aracer phone app. If you have a scope, you might be able to scope the tach line from ECU to the cluster. Assuming it's not a CAN signal or something stupid, that might tell you if it's the signal coming out of the ECU or the cluster itself.
If you decide to bin the cluster, let me know. I've been looking for a 'broken' cluster I can pull apart without worrying about damaging a good unit.
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