[bobski]

in the tach assembly there are 4 coils for this.

Did you just make that up?
The VSS outputs 4 pulses per revolution of the speedo cable. The tachometer reads 1 rpm per every 2 pulses per minute... For example, 2000 pulses per minute would indicate 1000 rpm.
My guess would be that the tachometer is simply wearing out. They're fairly easy to swap from any '88-91 Civic (the ones that have a tach anyway) or CRX.

 

[bobski]

I was under the impression that the tach received an electrical signal from the distributor, and that only the speedometer receives input from the speedo cable. :?:

Right... I was just comparing the two, trying to understand akira's line of thought. The speedo and VSS are driven by the speedometer cable (unless you have an SiR), while the tachometer is purely electrical, operating off a signal from the ignitor unit in the distributor.