A problem I just had was a cracked rotor. It is a rev limiting rotor and the little piece of plastic that holds the rear limiter contact in place (the little metal piece that sticks up) had cracked. When the rotor would spin the metal would move out and occasionally make contact causing the engine to stumble and the tach to drop. In my case it got worse and the car finally died.

I know this isn't helpful but I recommend going to an electromotive. When my sensor took a dump it just went to limp mode and still ran. Plus it has 3 coil packs so 1 bad coil won't leave you stranded. It really is the complicated-simple solution to antiquated and numerous ignition parts.