1979 Chevy El Camino 1979 El Camino lack of engine power

Engine Performance problem
1979 Chevy El Camino V8 Two Wheel Drive Automatic 100+ miles

My El Camino has power loss sparkplugs look like they have oil build up reving the engine sounds good until I hold it at a certian velocity then it starts to lack power I need to know what to look for and start eliminating possibilities? Thank you.