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Chevy engine trouble

My 1990 Chevy Silverado with 5.0 liter, 45k miles has started a problem that the guys in my diesel repair shop don’t know what to do. Last week it started to run rough, hesitating and lurching and not accelerating smoothly. It idles fine, or pretty well, but just doesn’t run smoothly with any kind of acceleration. I drive it back and forth to work just 1.5 miles during the week. I took it on vacation a week before, so it’s not from lack of long trips, it ran fine. a guy at my diesel repair shop put it on his Genesis scope and all it would code is the O2 sensor showed lean, but we could see it switching from rich to lean and so on. I thought vacuum leak, but couldn’t find one. PCV valve is good. EGR valve is ok. I changed the fuel filter yesterday and thought it ran a little better, but now has gone back to the rough running. Does anyone know what I can do myself to diagnose this? I am an apprentice diesel mech - 2 years, so I don’t have the knowledge, but I do have all the tools and shop. I really can’t afford to take this in to a gas garage. Besides, I need to learn how to do this.

check the fuel pressure. Sounds low.

Maybe a weak ignition part.
How old are the plugs, wires, coil(s), igniter (or whatever the ignition module is called)?
Does this have a distributor?