I would check the fuel tank vent for obstructions and replace the fuel filter. In that order. Sticking distributor timing advance weights mentioned by Michael is also something to check on.
Don't spin that motor to 6500 or even 6k in neutral. The valve springs aren't strong enough and you can get valve float, and possibly smack a valve into a piston at those speeds. I'd call 5k-5500 absolute MAX, but there's no reason to spin it anywhere near that fast.
My first guess based on the all of your symptoms is a clogged fuel filter.
1992 Supra Mariah - Red
PCM 351HO 285hp - PCM 1.23:1 Transmission
OJ XMP 4 Blade CNC 13x15.5RH
"People do weird things to boats" -Unknown