I have to agree with Mike on this one also. Usuall bucking as you put it is associated with bad ignition components. However since it seems to get better as the car warms up it is very possible the coolant sensor is malfunctioning causing the ECM to send bad signals to the iginition system. Start by at least checking everything athat mike said in the ignition system. If they seem to be in good shape then you may want to break out the DVM and download therepair manual that is stickied at the top of the forum page.