I had this same problem at one time. What your finding on the plugs is oil.
You said you added the intake and carb, was it recent? If so I suspect the intake gaskets did not seal. Sometimes you can pull the carb and if you find oil in the plenum then it's got a leak. Also in many cases guys will buy a new intake and they always seem to want to use fel pro gaskets. Certain fel pro gaskets are not for use with aluminum intakes and will not seal up. I always use the cheap paper hanging off the shelf Mr. Gasket jobs.