Pretty common in the Lucas world, but I've never seen it with a Merc. The ignition switch carries the load of most of the switched circuits. For those circuits with high current requirements, there will be a relay to relieve some of the load so the switch only has to power the relay.

This is problem could be internal to the switch itself, or it could be caused by a problem in one of the switched circuits. If it's intermittent, it's probably a downstream circuit rather than the switch. Where I would begin is by taking out the fuses, cleaning the fuse box contacts with a contact cleaner like Deoxit and a q-tip, and replacing them with new copper/ceramic fuses. Also remove each relay, clean the contacts, and replace. If that doesn't clear the problem, then you will have to go through the wiring chart, and test each switched circuit for high current draw, as well as testing the ignition switch itself for high resistance.