could you point me in the direction of where the earth wire mod needs to be made?? i have heard about it but havent been able to find the technical info on how and where it is...This could also be a main wiring loom problem. The 406 has this problem. I have fitted two main engine bay looms in 406s with this fault.
There is a procedure to fit supressed cables between the throttle and the ecu as a stop gap.
But it could also be the throttle sensor. but rare.