Did you have the MAF unplugged when installing the K&N kit???
If so try taking the neg side of battery off for about 30 mins, then when you put it back on let the car idle without touching anything, at least let it idle 15 mins.
Worth a try, I had a problem sounding just like that, just needed the computer to reset. Would have not found that problem if it wasnt for the guys here on this site.

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98 GT, Vortech T Trim with Power cooler, AFM Powerpipe, PI intake, Trickflow Plenum and TB, 90mm MAF, 255 Fuely 42lb shooters. Longtubes, H and Super 44s, Suspension and connectors, Driveshaft and gears, Spec flywheel and Clutch, Cobra brake swap. Steeda, MM and UPR stuffed in there too.