ricekiller

i have just recently swaped my old motor out and have a fresh rebuilt ls/vtech in the bay do to a stupid street race with a z06. for the first 2-3 weeks of driving it was a blust, but now a problem has occured. when i easy off the gas and try to go at a slower pace, the engine sames to sputter, and accelerate softy it sputter for a spite second then go smoothly. when i lift off the gas it sputter but when im completely off its smooth. what do u think is causing this annoying event to occur in my fresh motor. any comments appreciated.