Engine bogs down

a couple things come to mind, have you replaced all the filters yet (air, fuel, etc.)? another possibility is that the rings on your pistons or even just 1 piston are bad causing bad compression, the last thing that pops into my mind is that the rod bearings could be sticking from lack of lubrication caused by a bad oiling system or sludge in your oil. Now I don't know what all test have been run for sure because I don't know how you can simulate the conditions of driving with a load on the motor whil runing tests upon these systems you listed but I hope this helps.

Where are you reading fuel pressure at? Hopefully at the rails. Still sounds like a fuel filter is most likely culprit, or some other problem with the fuel system. Hell, I've had the exact same symptoms from getting bad gas, which is happening a lot more often nowadays. May even be an injector issue and sure you can check pulse width but thats just the electrical input, one may still be sticking or something. I would make sure fuel delivery is on par and find a way to test Air/Fuel ratio, find a quickie wideband tester or something :). Gotta remember, it IS a 12 year old car.