1. Hall effect or rpm sensor
2. General ignition fault
3. Bad Fuel Pressure Regulator
Next time it happens see if the tach drops to zero at any point when its jumping around (like pinned dead at the bottom of the tach while you're moving). If so its almots undoubtedly the ignition system and the hall or rpm sensor, and I would start there and work my way back to the coil, plugs and plug wires, and the ignition control unit on the driver's side fender.
You can read more about all this in the FAQ under the ignition and performance sections.
-rt
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