tach reading funny

my tach works, and all
but alot of times at idle it drops to zero....then as soon as i give it some gas it moves like normal and everythings fine, but back to idle and it zeros out..... occasionally it will read as it should right around 1,000 at idle, but most of the time its at a zero idle..... .has anyone seen this before?

Mine is doing the same thing on my 90. It said like 100 the other day when it was for sure way higher than that. I don't really know how it could be inaccurate though.