The revs dropping when the alternator loads with electrical accessories is normal. If that's a problem then tweak the idle up slightly.
I'm not too familiar with carbs that are not SU's, but fixed jet carbs often have an idle jet, which may have become partially blocked. Check your manual and see if there's an idle jet and where it is. Often they can be screwed out and blown through. If you have an in line fuel filter, fit a new one.
That's the simple place to start. It could be fuel vapourisation or an intermittent electrical fault but start with the basics!
See this site - two idle jets on this carb:
http://www.locust.org.uk/knowlegebase/weber/weber.htm
