Ok, you checked the fuel pressure but what is it? Can you rig up a longer hose so you can see what the pressure is while it's under load and the problem is happening?
Unlike the Vortec small block which won't even start if the pressure is below about 55, the big block has a different injector setup and will start and run even if it's down in the 30's. It wants 58-62 psi.
You could have a pump issue, regulator issue, or even something stupid like the little vacuum line between the regulator and intake could be bad, was cracked and leaking on mine. What sucks is the intake is in the way of seeing it/replacing it.
Distributor could be worn, pretty common with some miles, you'd be amazed how bad this can get and you won't get any codes. Usually you would expect to see a P1345 cam/crank sensor correlation error but not always. Pop the cap off and try moving the rotor side to side checking for bushing wear on the shaft. If it doesn't move try turning the rotor either direction until it pops up a little then try side to side again while it's up.
If your distributor has been removed or loose ever for any reason, has the cam retard adjustment been done? Must be within +/- 2 degrees of zero while holding RPM's to at least 1000; you can be a whole lot off and not set a code but it will run like crap. And I'm not talking about timing; these can't be set with a timing light. Gotta have a scanner that is capable of showing the cam retard angle.