Help me trace, lots of smoke, no power
 

1991 6.2 GM in a suburban. This engine has no EGR. Starts up no problem, idles smooth, no misfire ever. When you drive, it will loose all power, its intermittent and happens at different RPMs. When it happens it will smoke heavily. Engine has mechanical lift pump. Makes a consistent 4psi. Fuel filter (square one) is new. Problem came on all of a sudden. I'm thinking maybe that the advance cam in the injection pump is getting stuck. What do you think? If it is, can the pump be repaired without special calibration tools? I do a lot of late model diesel work but have limited experience with these old all mechanical GM units. Setup seems really simple and should be easy to figure out.

Thanks for the help!

You should have more then 4 psi from your lift pump I'm pretty sure that 7-9 psi is stock mone is running 9 to 14 and when I had the problem you are describing that's what it was a bad lift pump

Mechanical LP's don't typically deliver the same pressures as electric LP's.
Excess smoke tends to make me think fuel quantity is acceptable.

That's what I was thinking. Spec is 5-8psi, so I'm not quite making that but it doesn't perform like lack of fuel. It doesn't cackle when it happen either, the engine actually gets pretty quiet. My feeling is its not advancing but I don't know how to tell for sure or if I can open up the pump. Rebuilt pumps aren't cheap so I don't want to through one at it without a better hunch that the problem lies there.

Try some two cycle oil in the fuel tank. Start with about 16 ounces. Drive around, 2-5 miles, and see if it helps.