Someone who drives a MK2 Golf mentioned a problem like this. The MK2 Golf (I think) uses the same k-jetronic (?) as on the mechanically injected Saabs. He said this would happen when water got into the ECU. Don’t know if that helps?
The rev counter takes is signal from the ignition amp attached to the nearside inner wing, which in turn gets its signal from the hall effect unit in the dizzy. I would check connections between these two units initially.
I assume nothing was touched at the ECU in the drivers footwell?