sounds like the igniter. make note of which cylinder is dropped, swap the coils and leads from one side to the other.
If the problem stays with the same side cylinder then it is the igniter. I went though the above and more to isolate it. The problem started out very intermitant and got more and more frequent. Triumph replaced the igniter under warrantee even though it was a ways past the 2 years. I have to say they do stand behind their bikes.

05 speedmaster - 1100cc, 11:1 racing pistons, Carillo rods, thunderbike cams, ported and polished head, 2mm over intake and exhaust valves, Barnett kevlar clutch, scepter pipes, oversize manifolds, 45mm HSR's, TTP stage 4 firestarter