Hi Roger, and Andy and Sylvia,
I noticed Klaus hasn't responded, he's probably off somewhere, so I thought I'd throw in my two - well, one - cents worth:
Some questions and hints for you to help us solve this "interesting"

problem:
Have you checked ALL grounds (earths) and connections - disconnect, scrape clean if possible, especially battery terminals and main grounds and coil grounds? Spray with contact cleaner too. Clean ignition switch, disconnect sidestand/clutch switches if they exist.
Has the plug cap and wire been replaced since the problem started? (measuring ok isn't good enough - can fail under stress - and note that a hot engine requires less electrical juice to start than a cold one, so everything must be better in order to start cold - therefore electrical connection suspicions)
But just in case:
Have you checked for air leaks in the inlet manifold/carb mounting?
Does it burn any oil now? How is the compression - has it been tested, and what are the numbers hot/cold and with a little oil squirted in the cylinder? Are they ok or not?
If all that checks out:
Is there another bike available to swap parts with?
If so I'd start with carburetor complete, (cause it's easy and eliminates a whole area)then coil, and other ignition components, AFTER a thorough check of grounds.
Hope that helps!
------------------
Grant Johnson
Share the Dream!
at:
www.HorizonsUnlimited.com