Originally Posted by dojj

i'd say it was more along the lines of the pump failing, you aren't using enough fuel in the lower gears so the supply will cope for a few seconds, but when you try to pump enough fuel for the longer duration it's not getting there and it's running lean instead

lowering the boost means you don't need as much fuel and so it's only doing it in 5th

easy to check before you get it remapped or set up again as it could just be the wiring at fault or a blocked filter