had the same problem with my '92 745 with regina FI.

I did the following to solve my problem:

1)cleaned all the electrical connectors to all the sensors,
2)cleaned and tightened all the ground connectors, specially the FI grounds,
3)cleaned and tightened the FI red cables at the + side of battery, pay special attention at the crimpers, looking for corrosion.(mine had alot of a green powdery substance.

and finally, replace the rpm sensor if it's the original (yellow plastic band by the electrical connector). New sensors have a white band an are more reliable.

Note that the first 3 recs cost you nothing, the sensor is about $35.