I would, off hand, suggest fuel pump. I have also heard of several stories where the car is fine one minute, go into a store, come out, then there is a miss that will develop into a bucking like you describe.
I had the same issue right after Kim and I bought our Mystique. Started out not so very often at highway speeds only, but soon started even below 2500 RPM. Turned out to be plugs and wires in our case.
I would suggest, if you haven't already, checking out this thread...
To those who have bucking.... There seems to be a lot of different senerios you could play with here.
If you have already checked this thread and the few others that have turned up here in the past month or so, I would see if you could rent a fuel pressure gauage from an auto parts store and check that.
Better yet! Check that coil pack connection! Remember that little joke someone (who shall remain nameless at this point!!
) pulled on you not too long ago. Might not have reconnected it very well and it is just now starting to work loose again. Just a thought anyway.