I just bought a 2000 440Z and it will start and idle just fine, and when you're on the gas hard it runs like a champ. but when you get off the gas it backfires some of the time and when just riding at a steady pace it will try to stall until you let off the gas and it will take off again. if you hold on the gas it will stall but if you let off just before it dies it will rev back up and sit and idle just fine. I have no idea what the problem would be any ideas?