Had identically sounding symptoms and suffered for 2+ years while unsuccessfully trying out most things mentioned in various forums. Finally tracked it down to a very slight build up of oily cr@p inside the dizzy cap. Resolved it by a quick spray of a solvent called something like 'Electrical Contact Cleaner' and wiped round entire cap and rotor arm 'til squeaky clean. Problem immediately disappeared and hasn't returned (4+ months now!).

Someone once posted an explanation that the oily film coating the inside of my cap was probably vaporising at a certain temperature and somehow affecting the sparking. This was why my engine would idle perfectly from cold but with a kind of 'hiccup' as temp hit 60-70deg.

Hope your problem turns out to be something so simple, and you don't go through $$s (as I did) to find the answer.

Best of luck,
Neil