You're detonating. White plugs = lean condition in the combustion chamber. Change your fuel filter, and if you didn't upgrade the ECU, fuel line at the rail, or the injectors when you did your swap, then there could be much bigger restrictions than your fuel filter to your fuel flow.

Usually you'd experience that at WOT, though, not off-throttle unless your TPS sensor either isn't hooked up or is damaged. :thinking: