You first need to tell a little more about the history of maintence please. If you have checked the wires, caps, rotors, and plugs, I would start with possibly a vaccuum leak somewhere. It has been known to be a little one to cause the symptoms you are describing. Furthermore, there have been instance where the IAC (idle control valve) is not working like it should, as well as the MAF (mass air flow sensor) is dirty or not working at all. I had the same problem, 92 SC400, and it was it was bucking when you off of the throttle at ANY speeds. It drove me nuts. I first thought it was the rear end bolts from the axles loose again, but it wasnt. It sounded like it was from under the car.
I ended up cleaning the MAF sensor, adjusting the TPS (throttle position sensor), and the TRAC sensor. It bucked for a second or two when I first put everything back together. Then I floored it a couple of times and it seems to have worked out, no more bucking. I just experienced this the last three days and did the above things this afternoon, and it drives fine now. Silky smooth. I hope this has given you a direction to attack this, and let us know what you find out.
Is there a CEL or trac light on or flickering?