Would be curious to hear what they have to say.
For grins, I just loaded stand alone K5, pulled up Traditional multi Full Ens Sustain Orchestrated.nkm and played a few notes. Everything sounded fine Until I played too many notes in too short of time frame and then it was breaking up like an mf'er. (see what I did there...)
K5 showed that each single key press plays 12 notes so just playing a chromatic scale things would start breaking up after about 1 octave. The K5 CPU meter was pegged and windows 7 also showed core 1 was pegged even though it was hitting 3 other cores pretty hard.
In K5 options I changed the CPU overload protection from none? to medium strength and that helped by killing old notes sooner but if you play faster it breaks up again. Haven't try the 'Strict' setting yet. (on my to do too list today)
Fanfare sounds great but the CPU hit really limits the usage of some of the library. Checked the spec's page and they have a core2duo listed...
Sorry I wasn't of more help but at least I am now able to confirm your initial issues as valid here also.