Well, the tachometer does run off of the ignition module, and if the tach drops to 0 only when it misses, there definitely is a connection. Ignition modules do go bad. Last time I checked for an '89 they were more like $2000 for new, around $500 for rebuilt, and $100 on up for used. And all should be available somewhere in the US, though there are many different part numbers for these modules, so you need to get the one to fit your car.

But if it is the module, I would think that it would quit completely and not just miss, especially if the tach drops to 0. Maybe there is a short in the tach circuit or tach itself. If I remember right there is a junction block on the fender well where the tach wire runs to and can be quickly disconnected. I would definitely rule out the tach before replacing the module.

The only other part I can think of that might make the tach drop to 0 is the ignition pickup sensor on the flywheel, but then again, the engine would stop completely if it failed.

If you install a new or used module, don't forget to apply a coat of heat conducting paste, as well as the plastic foil, to the back of the module.