Hi,
First off i`d give this a try:-
If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1150 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture.
If this doesn`t help then check the battery is in really good condition as a slightly weak one can cause havoc with the ignition circuit on the triumphs.
If not luck with either of these then it`ll be a case of replacing the coils.
Cheers, Matt