OP, yes I've had this with my PWTS and CSP. Not on GT5 but with the Forza 4 demo and C.A.R.S on PC. Drops to about 95% throttle and the only way to clear is to turn the wheel off and on. Note I've only experienced this after extended sessions, 2+ hours which makes me think maybe something in the wheel is getting over heated and a reboot clears it?
If I get it again I intend to test the pedals by connecting them directly to my PC and see if I get full travel.
Thought I should update this to say my problem has been fixed by replacing the PCB/sensor that is mounted on the accelerator pedal. As Fanatec managed to send me two replacement kits (which include both accelerator and clutch) I'm covered should I be unlucky enough to suffer the same fault again in the future.