I had a smilar problem a long time ago.
I had recently fitted a dawes device and some hks plugs, went drag racing and it kept losing boost in 4th geard, then it started doing it in all gears until it just misfired.
I suspent it might have been faulty dumpvalve as it blew off the pipe few hours before the misfure happened. But i put the enging beack to standard and everything was fine again.