Under load at around 2500 rpm this just dies as though ignition has been turned off then when revs drop it revs up again with a bang and repeats the cycle.

No fault codes, no misfires just a clean cut out

We can not make this happen in workshop only on road test

We have checked live data throughout rev range no apparent glitches
4 gas readings are spot on at idle and throughout rev range
scoped o2 sensors,tps etc all appear to be performing correctly
checked fuel pressure and flow
in desperation we tried known good crankshaft,camshaft,maf sensors no difference
dropped exhaust in case of blockage cleaned out throttle body
nothing alters the fault any help on this one would be much appreciated