AS350B2 Power Check
I would perform a power check at 97%, 98%, 99%, and 100%. Look for a trend to see if you are generating progressively less power at higher settings, or 'better' power the higher the Ng you pull.
The 'climb power check' you are referring to is the 100% Ng setting. You have to climb in order to perform it. Try doing it straight and level and you'll see what I mean. Be careful not to exceed any airspeed/power setting limitations.
Pull the p2 filter and clean it. Perform an engine wash, and follow it with a thorough rinse. Pull all the stainless steel lines on the engine and check the flares for any sign of contamination, especially the fuel line. Run the engine with the cowl open to make sure there isn't anything leaking around any of the lines.
What has Eurocopter said about any of this? Have you spoken with a tech rep yet?