I also have had engines quit suddenly when going from 1/2 throttle to full, and found a few reasons:

1) plug The #6 novarossi can be replaced by OS#8, OS-F or Enya #3

2) compression ratio. A talk with MVVS taught me, that their engines are adapted for the US market. This means that lower compression ratios are used at the moment, to cater for added nitro. This is done by adding squish deck clearance. Not the best solution IMHO, which brings us to

3) nitro content, as David already found out. Adding nitro in the modern engines largely solves this problem.

4) When the engine flames out, it is due to fluid fuel surpluss at the plug. Leaning the idle needle to the minimum is the solution, combined with the type of plug.
Using the right plug allows one to reduce needle settings considerably, so that can be used as an indicator.

Using MVVS supplies and some elbow grease, I have constructed a small batch of .25 (3.99cc) Aces 1/12 combat engines, with a squish deck height of 0.2mm, and effective CR of 9.2. These engines run safely at nitro contents up to 15%, and do not exhibit the flame-out that David experienced. Such a tight squish height however is not feasible in MVVS production due to the number of toleranced dimensions to cope with. It only can be accomplishes using the now abandoned system with combustion chamber buttons, which are deck height classed to allow engine blueprinting.
Ah... the good ol' days.