Tachometer Problem

[xxalexx at ix.netcom.com]

> My tachometer rises with engine speed up to about 3000 RPM and then seems
> to stop or randomly increase a while the engine is accelerating fine.  I
> have recently changed the coil to a MSD coil.  I have heard of this
> happening if you installed the MSD 6 ignition, but I don't have one of
> those.  I then re-installed the stock coil and the problem goes away.  I
> have noticed on occasion that the tachometer seems to slow down past 4000
> RPM with the stock coil.  Any ideas?  I checked the service manual but it
> seems that if the tachometer works at all they say to replace the
> "Electronic Spark Timing Computer" whatever that is.  Is this the module in
> the distributor?  Thanks for any input.
> 
Could be a filtering problem, look at signal with scope,
and or try some caps.
Is there any universal tach pickup circuits anywhere?
alex