Sounds kinda like my 2000 SSEI.
It only did it a few times now and then, at first around 40 mph but eventually up to like 65 or so, it would start studdering or lurching.
It then started doing it real bad all the time. (right after I kinda got onto the the gas pedal passin some slow traffic.)
My brother and I thought for sure it was something with the transmission because it only did it once it shifted past 2nd gear.
I took it in to the local tranny shop and he thought it needed a tune up. Replaced plugs, wires, fuel filter, coil pack, and even the TPS sensor. It helped but didn't remedy the problem.
So he checked it out some more and eventually figured out it was the tourqe converter. So that finally fixed the problem (a grand in parts and labor)