I have a 1980 280CE chassis # WDV 1230531209364 that is not firing on #5 I have changed wires, plugs and blown out the gas lines. While doing compression test it hit 150 then the engine started and the compression dropped between 30 to 60. I checked the others and they all stay at or around 150 while running. What could be doing this?

Thanks for all of your input!!