I had the same problem a few months ago, ended up gutting my cat because of it, but now I'm glad I did, gained some good down low from it, but my problem was a coolant sensor. Car would buck at anything over 1/4 throttle and would just bog until it would stall, sometimes when I waited to start my car it would fire up and would be fine for a few minutes till it heated up again, then it would crap itself again. The sensor is right under neath your intake elbow, if there is a check engine light on when this happens chances are it might be that sensor, about $15 or so and you should be all set with replacing it. Shortly, more like as soon as I replaced the coolant sensor, after my O2 sensors went, might as well check up or replace any cheap sensors while you're at it, if you haven't already.