Since it's already plugged up twice in the last two years I would check this 1st. Fuel & air demand is the highest when the engine is working hard. If the problem only occurs in these conditions, then I'd be looking for restrictions in both of these areas. Things that can cause these issues are: plugged fuel filter, failing fuel pump, pinched fuel line, restricted intake air (filter or debris in air box), or plugged up cat converter. Another thing to check would be ignition coil (although I haven't heard of these failing on a Previa yet), pick-up coil (in the distributor), or igniter. There are other things that can potentially cause this problem, but these are the big ones.

Fuel & air issues usually won't throw codes, but electronic issues often will. Check the ECU for codes as this might give you some valuable clues. Good luck. Tim