The Bonneville tachos are notorious for bouncing around at 4000 or so RPM. Many of them do it, including mine. It's not too bad on mine though, so it doesn't bother me.
Yours seems out of control, hopefully it's covered by warranty.
2005 Bonneville Blue 790cc, AI removed, Staintunes RC, Uni filter, no snorkel, 118/40/NBZT "Thruxton" needles/1 shim/3 turns, tachometer, Ikon 7610s, Ricor Intiminators, Dunlop GT501s, D9 gauge panel.