Jumpy Tach

[Alpine TSi]

92 GVR-4 with about 163K, no pertinent mods.

As of late I been seeing that that during acceleration(both hard and regular) that my tach needle likes to skip about 400 rpm on occasion. It doesn't always do it but usually only under load(boost) and while accelerating. Nothing noticable happens when it skips, just the tach stops for a brief seconds then skips forward. No difference in engine performance or overall power of the pull.

What could this be?

Thanks
Matt