unplug the vacuum line going into the top of the BBV and cap that line.

If your pressure returns then its not the bbv.

And temps have a massive effect on PSI.

Its probably shifting at 6000, then it slips up to 6200.

Valve float can happen up there. Unlikely on stock valvetrain though.

But these things make no power after 5500 RPM anyways. At least without valvetrain mods.

Get a wideband street tune by someone who knows these cars.