intermittent fault on 807?

whilst driving my 807, with the throttle in a constant position, the engine behaves as though the throttle is being lifted on and off with the car losing power and then surging forward. There is no pattern to when this happens, engine hot or cold, travelling slowing or fast, tank full or not and then after sometimes a couple of day sometimes weeks, the faults rectifies itself and the car runs well. It does not show any fault code nor a fault when plugged into AA diagnostic computer and when I take it to the garage the fault then seems to disappear, bit like toothache. Has anyone come across anything like this. I am at my wits end.