I take it that it does this in all gears at 6000rpm? If so, then I would have thought that a bad TPS is unlikely. I can't see that you'll always hit 6000 rpm at the same throttle position in all gears.

Sounds like you've pretty much checked everything. Don't want to harp on about chips but as it's only a 2 minute job, a chip change has got to be worth a try.

I would suggest going with the single injector conversion. We've had a few do the same thing in the upper rpm range and the conversion solved the problem.