Sounds to me like I would temporarily disconnect the NOS kit to trouble shoot the engine. Disconnect it, pull the pcm memory fuse, fire it up and let it learn for about an hour of driving (if possible). If the problem happens again then perhaps you will want to use a wideband O2 to see what is happening. Is your fuel rail a return style? Check to make sure your fuel pressure regulator is plugged in and has good vacuum. If unplugged it will run lean under accel.

warmonger

Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black