Check this thread out:

http://www.rcuniverse.com/forum/m_88...tm.htm#8844410

These engines are simple combustion machines.
Engines that operate consistently well are the ones that keep the three elements of combustion inaltered during long periods of time: heat, oxigen and fuel.
I believe that fuel is the element over which we have more influence regarding chemical composition (type-brand-degradation of nitromethane due to Sun light), humidity content (age and exposure to atmosphere), volume fed to the engine for the whole range of rpm's (needle valves adjustment), cleanness (filters, many filters, as many as possible).

By keeping the same brand-type of glow plug and combustible, as well as all the hardware elements in top condition, our variables reduce to two needle valves.
Once we hit the sweet spot, we will enjoy reliable high and low rpm's (ideal proportion of air-fuel for each of both regimes) for as long as the conditions remain.
Changes in air temperature and density may require small adjustments.

Quality engines are normally trouble free from the factory.
Our little mistakes in break-in, operation and tunning may make them a pain rather than a joy.