All sensor grounds go back to the ECU, but I will double check the TPS ground as soon as I can. When I solved this issue, that was my best guess as to why the TPS% changed when the car was IDLE versus OFF.Where exactly is the sensor ground on the TPS connected? It needs to be wired to the ECU sensor return; grounding it elsewhere can produce the exact same problem you describe.
I just haven't had the time to take a look at it yet.
I'll let you know what I find.