1998 Honda Civic Losing Power under 40

Our 1998 Honda Civic loses power every time you drive it at less than 40 mph. The mechanic found ruined spark plugs and replaced them. The car still did it. While they were looking for answers, 2 more cars were brought in doing the same thing. After looking at all 3 they replaced the distributors. No change. They have changed the engine, transmission and anything else they could on a computer. No problems and also still no fix. Two suggestions we have received from others is the oxygen sensor and vapor lock. Does anyone have any ideas?