When my car gets up to operating temperature,it want's to stutter as if it was misfiring.It does it when it's cold also,but is worse after the car warms up.Some times it gets to the point I think the car is going to stop.After I get up to 45+mph it stops.I'm not showing a check engine light and my scanner isn't showing any codes.The opti,plugs and wires were replaced a year or so ago.Any ideas as to where to start looking? Thanks