Did this happen all of a sudden? Or did you do some work and then the problem occurred?
Mk2 ignition switches are common to fail and have different symptoms to show that its faulty, for your problem to happen you either have some bad wires to the ignition switch (bare and touching!) or the ignition switch itself. Cost £10, but requires some tools to do it.
I wouldn't change it just yet as i'm sure someone has an idea of this?... I hope!