I have a 2008 Speed3 and I am having a similar issue. However, it is somewhat different ,the stumbling occurs only under load, does not have to be WOT in fact it happens during smooth acceleration, never when stationary. I can sit in my driveway/garage and rev the snot out of it and it never misses a beat, as soon as I get to the end of my street and turn onto the main rd, shift 1st to 2nd no issue up to 3rd and gently accelerate and then bang, major fuel cut, lasts for about 3 seconds and bang back to full power. This goes on continuously until I slow right down let it idle for a second or two and accelerate gently again and then it happens again. I have done the relief valve test and monitored the fuel rail pressures and they are normal. However I did notice that the injector pulse width goes to zero as soon as it happens which would explain the sudden fuel cut off. There are no fault codes, no pre-stumble misfires just complete fuel cut. I also monitored the TPS signals and they were normal. I am using a Foxwell NT530 scanner which I primarily use for my Beemer and am in the process of sell the Speed3 but need to fix this issue or no-one will buy it. I am not sure if I can grab a datalog with the foxwell so I am trying to go through potential causes one at a time but I was hoping that someone may have come across this problem and figured it out. Any help would be gratefully appreciated.
I forgot to mention that I have already cleaned the spill valve.
