My tacho is also a law unto itself.
Sometimes it sits at zero when the engine is ticking over and most of the time just sits there flicking around between 4-5K regardless of what the revs are. It sometimes 'improves' when I give it a fistful.
The loose battery would explain it. My tacho has only gone mental since I sent the bike down the road on it's right hand side at Rockingham. That would've loosened the nuts...I know it loosened mine.
