Disconnect your battery for 20 mins or so to let your ECU self-adapt to the new gas-air mixture you're getting with your K&N. If that doesn't work, I'd suspect that "coincidently" you're dealing with another issue. You can always put the old filter back in and see if it goes back to normal to see ...

HI EVERYONE, from sunny manchester england!!, ive just fitted a k&n kit to my 850 t5 2.3, and occasionaly when i kickdown the car spulutters and farts rather than take off!!, but its only done this since ive fitted the k&n, can anyone help???, many thanks, kev