tachometer issue

my tach has been intermittenly acting strange recently, it wil rapidly jump around at idle in drive, there isnt any actual change in RPMs or voltage according to the scangauge, of course since this video was taken, it hasnt happened again, vx doesnt even shutter, just looks strange

http://www.youtube.com/watch?v=Y2U1yGQEaVQ

I'd be more worried about that horrendous noise your motor was making in the video...:smilewink

haha the sound on my computer is usually off.. didnt even notice that sound.. wonder what it is:_confused

Hi rev counter jumping around is either alternator or crank shaft sender
cheers
Steve