Tippy,
In Frank's case, I feel pretty sure he would have blown his motor if he were seeing off-boost timing in that scenario. 4000rpms in vacuum he probably has low to mid 30's ignition advance - the car has likely been tuned for half that advance at 10psi of boost.

I think it's more likely Frank is seeing a brief boost spike and the ECU has been setup to cut ignition. It's too strange that the problem is repeatable only in those conditions and doesn't show itself elsewhere.

Update: appears the Motec connector going into the expander has a loose connection and is intermittent. When I changed the placement of the unit (straightened out the cable) no more issue after repeated drives. The tuner confirmed that this sometimes happens during termination of the connector and results in the ghost-like misfire (ign cut). The maps were fine at all RPMs and loads.

Will re-terminate come the springtime. Thanks a lot guys for all your suggestions. Problems really help you learn about the system, which is part of the experience :)