[Miyoko Rei Hinata]

Is it just the tach jumping around or is the engine revs jumping around too? If its just the tach, I would go would with blown caps or cracked joints on the board. Re-flowing the solder and replacing bad caps should fix it, or grab another tach from a salvage yard.

If the engine is jumping with the tach, I would say rev limiter setting. Stock LT1 I think is set around 5900, but your showing a tune so maybe it got changed? When the LT1 PCM hits rev limiter, it cuts fuel instead of timing like on some other vehicles.