Your problem description is a bit cryptic " my car lost power for just enough time for the tach and speedo to fall to zero for a fractional second". Is this accurate? Did the engine actually stall, bringing the car to a stop? Did the speedo go to zero while the car was still moving? Did the tach drop rapidly to zero even though the engine kept running?

If tach dropped to zero very quickly even before the engine wound down this would mean you lost igntion pulses to the coil. Most likely cause would be connection for the hall sensor at the distributor. If connector is really bad it can jiggle around and chafe the wires. Also check for loses wires around the power stage and coil.

The above would not cause the speedo to drop to zero unless the car actually came to a stop. If speedo went to zero with car still moving you have some other electrical problem such as bad ignition switch or other loose connection somewhere.
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David Hunter