I recently replaced a head on my 91 Ford Explorer 4.0 liter engine due to a blown head gasket. Now the engine starts up fine and begins by idling smoothly in the driveway. Then, after about a minute, it starts to stumble then shudders, catches itself and again idles smoothly. When I begin driving it will not accelerate properly for a couple of blocks but then kicks in and runs fine. What could be causing this?