I would bet on the throttle actuator being bad. On your engine, and mine, the throttle linkage actually moves a potentiometer rather than directly opening the throttle plate. The potentiometer alters the current flow to a motor which actually opens the throttle valve. This is so that when the ASR senses wheelspin and wants to cut the throttle, it can do so. As you know, if the rear tires are spinning, you can mash the accelerator to the floor and the engine speed won't increase.
When this assembly goes bad, the signs are: ASR light comes on and throttle assembly goes into "limp home" mode whereby you get no throttle response for the first 5/6ths of accelerator movement, then you do get some throttle response for the last 1/6th.
Mine has done this twice, and both times reset itself when I shut down and restarted. I'm probably living on borrowed time.
If you do a search on "throttle actuator" you will get some add'l info.
So far, it doesn't seem possible to rebuild these units. Unfortunate, since it's probably a $10 part but you have to replace the $800 assembly.