Ive got a 2003 WRX, and im having a problem!

It is fine on start up, at least i think it is! It might be i dont notice it because im taking it easy! Anyway, once its warmed up and i am on the throttle for any length of time, it feels like i hit a flat spot, but it doesnt pull through it! Almost as if its not boosting! If, when driving i kill the ignition and knock it back on, problem solved. Well for a short while anyway!

The car is standard apart from a cone K&N and a tubular manifold (non equal length).

Can someone please help?

Cheers in advance...