Had a similar problem on my MK3 3i.

Turned out to be bad contacts on the ignition relay.......would occaisionally lose heater blower/wipers/rev counter etc.

Cleaning the contacts didn't work, wedging/fiddling the relay did.
I ended up coating the spade connectors on the relay base with a thin tinning of solder..about 1mm thick to tighten the connections it plugs into.

It's been fine for best part of a year now.

Hope that helps.

Ant