As your running it on the bench and not getting some good temperature into the motor, the crankcase is not heating up enough to properly vaporise the fuel held within it - so your crankcase is in a "rich state" at idle.
With the first couple of throttle blips this overly rich mixture which holds droplets of fuel as opposed to a nice mist of vaporised fuel/air mix will make the motor run rich and blubbery. After a few blips the excess fuel will be cleared from the crankcase and a leaner fuel/air mix will be available to the motor. This is why it clears after a few blips.

It sound like you are running a little too rich - probably top and bottom.
This rich setting is causing excess fuel to build up in the crankcase as the engine sits there idling away. Sooner rather than later this mixture will become so rich the motor basically floods itself (the revs drop) and it will eventually flame out as you describe. The colder plug may well require slightly leaner settings than before too.
Make sure your idle gap is set at .5mm. If the motor won't idle at this setting then lean the bottom end needle until the idle speed comes up to a satisfactory level - getting the relationship between the idle air gap and the bottom end needle is critical to achieving a good tune on any nitro motor.
Always make sure you run the motor up to a good temp before you tune - tuning a cold motor is a recipe for disaster, because of the low crankcase temperature / rich mixture situation mentioned previously.
Hope that helps