its a misfire most likely, does it shake at all in overdrive too or studder a little at take off or when in gear? check plugs wire coilpacks even the ignition module(plate that coilpacks plug into) and also, make sure it isnt loosin coolant, that could cause it to run a little rough.
i had one come in running on only 2 cylinders. cam cracked right apart. As i stated though check the normal wear items first as the cracked cam is always a last resort. Though not hard to check at all. is it throwing codes at all? consuming alot of fuel?