I had a similar problem with my 97 D1. Sometimes when the driving lights were switched on it would kill power to some of the other lights. I removed the switch and noticed there are some contact points that had built up some bumps on them and the current was not allowed to travel properly. I ground these down with a dremel tool then polished the contacts. This seems to have fixed the problem for the time being. Putting it back together can take some time if you don't get everything lined up. Make sure you pay attention to how things came out and watch out for any small parts.
Skyler Neibaur
1997 Discovery SE7