OK guys, finally sorted.

Not AFM (I tried 3 different ones in the process), took it to RMS in the end as I had no idea.

Ended up being the O2 sensor! argh.

I didn't consider this as I thought the O2 sensor isn't used on wide open throttle and the ECU just falls back to it's maps and the AFM readings. He said it was running really rich (hence the bogging down) so even with the foot down it probably still had all that fuel to burn off first (I tended not to keep the foot down anyways once it started hesitating so I didn't blow the engine, haha).