Intermittent Power Loss in 1989 Chevy G10 Van with 305

For 10K the van loses power intermittently for 1 to 10 seconds then comes
back to normal power. It happens at all speeds from thruway to idle. A
variety of shops including a Chevy dealer have failed to eliminate the
problem. The following have been checked or replaced: fuel pump pressure
checked, fuel filter replaced, spark plugs and wires replaced.

Does anyone have experience with this specific probem on this vehicle engine
or similar?

Any guidance would be appreciated.