Jumping Tacho

[PG325]

The car came out of hibernation a few weeks ago, since then, I've noticed that the rev counter jumps instead of smoothly moving throughout the range. I don't think this is SI board battery related, as I recall these either working or not, but I'm wondering what could cause it to jump, any ideas?

[Contours]

Mine did the same until I replaced the batteries or it may be just a bad connection to the battery terminals within the cluster.