Hello there, well i'm sure the guy's who have alraedy replied are pretty clued up bikers and there reasons are good.
I had a problem like this with a Honda 400/4, the fault was only down to the spark plugs of all things.
2 of them were missing sparks when the motor was warmed up and under more combustion pressure.
Also if the HT lead is at fault i found out that when the motor has warmed up it can cause 'leakage' sparks to 'track out' from the lead to other area's of the bike, mostly arcing out to the frame or cylinder head. I suppose you only need a miss fire or 2 together to cause cutting out on a big single, bares a thought.
A test i did was to fire up the bike at night on my really dark back garden and wait to see if there's tracking , arcing from the HT lead.
I also found out in time that the resistor plug cap can go faulty too. Cheap things to replace are the plug and cap and then check the HT lead as i said above. A good quilaty 8-10 mm bore tubeing won't go a miss over the HT lead to solve a tracking problem.
Just my expirences in this.
Cheers Splint.