I have had a similar problem with my '67 102. It ran for 2 summers no problem then....After about 20-30 minutes of riding it will cut out when I reduce speed (ie stop at a light). It will start back up but will keep cutting out for a while. If you let it sit for a good amount of time it will run again for about the same amount then start cutting out.
At first I thought it was a fuel problem (specifically that I had replaced the gas cap with one that did not have an air hole). Tried a substitute- nope, not that.
I cleaned out the carb and air filter, checked the line...nothing.
What I and anyone mechanically inclined that I know have started to believe is it's an electrical problem. I'm going to try replacing the plug, and/or put in a cooler running spark plug to see if that will help. If anyone else has any ideas, let me know.
Hopefully it's not the coils because I have no idea where I would find new ones.
Good luck all!