Now. that does seem strange. If that is the case it might blow the accelerator theory out of the water. Seems it could now be ECU related. its as though the ECU is losing its calibration. I totally guessing now but could this be a bad connection. Have you checked all the plugs onto the ECU and accelerator ? Have you had any ingress of water ?
At least you can get your stop start to work, since I changed my starter the only time when my stop start works is when I'm towing my caravan, when I really don't need it. Typical Antara. my car drives me insane sometimes, well most times, but I love towing with it. Its a perfect towcar.