I have an 88 BII auto, with a 2.9. On the interstate it has been seeming to not have much power, I have a hard time keeping it in overdrive, and it seems to want to lose speed. It also wanted to "buck" or sputter at about that 55 mph shift to OD. Anytime (on the interstate) when I would even drive up even "not so big" hills I would lose at least 15+ mph. I put on a new cap and rotor and wires thinking it might be missing. It ran alittle better, but still does all the same things. I have run injector cleaner through the tank, with little or no improvement. Today I checked the fuel pressure, thinking it may be a regulator, and it held steady at 30 lbs. even after I shut it off. When I revved it, it went to about 37 or 38. Then I changed the fuel filter. The gas that came out of the fuel filter was very dirty and blackish looking, (pooring it out from either side, dirty and black) tested pressure. Still at 30 and 37 on revving. took it for a test drive and still lacks the power. If I accelerate at basically full throttle or close to it, it will occasionally, un-sputter and accelerate decently quickly, before bogging back down. My question is do you think it might be the fuel pressure regulator or one of the pumps, and if so, then which pump? p.s. I dont notice any black smoke upon start up, but it has been getting bad gas mileage also. When idling it runs decent with an occasional stumble every 15 or 20 seconds)
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It seems your fuel pressure is at acceptable levels.

Are you saying the fuel filter was spilling dirty fuel from both sides? This is just a guess, but it seems like whatever got into the filter may have gotten past it, and got into the fuel injectors. They have a final mesh filter just inside their inlets that can trap debris. If crud got past the primary filter, that's where they'll end up. They're not fully clogged yet, but the crud is preventing full flow, so the engine simply can not generate power, so it will peter out under heavy load. Since these are solids, no amount of chemical cleaners will get rid of them. I've only been able to clean injectors in this condition by pulling them out and reverse-flushing them. I used a 12V battery to activate the injector while shooting carb cleaner in from the pintle end.