I had a problem like that - that stumped me for quite a while until by accident, I found that one side of the fuse holder was making intermittent contact...sliding in and out of the plastic holder. I fixed it and no more miss.
Also, in case that isn't it and per John Healy, long time Triumph mechanic and parts distributor, etc. etc. as well as the moderator for the Triumph forum on BritBike.com:
"One Rule of Thumb I have found to be handy diagnosing these kinds of problems is:
If it happens at the same THROTTLE OPENING it is usually carburetion.
If it happens at the same RPM it is usually ignition.
It doesn't always work out that way, but I find it a very good way of approaching a given problem."
Also, in case that isn't it and per John Healy, long time Triumph mechanic and parts distributor, etc. etc. as well as the moderator for the Triumph forum on BritBike.com:
"One Rule of Thumb I have found to be handy diagnosing these kinds of problems is:
If it happens at the same THROTTLE OPENING it is usually carburetion.
If it happens at the same RPM it is usually ignition.
It doesn't always work out that way, but I find it a very good way of approaching a given problem."