Coilpacks. If it's popping under full load and loosing boost I think that's most likely what it's going to be. Does it doing driving off boost or even under 4000 grand? Give it a try.
Or... long shot, problem with your actuator maybe. Check the seals.
Could even be clutch, although, there shouldn't be any reason for the pop in that case.
I think coilpacks is your best bet mate. Common problem:D