1974 Dart Police Sedan - A day in the life

my slant six duster was overheating on the highway. the rad was blocked up. it was ok for a while and would do it after a few trips. id flush every 3 or 4 times i drove it and black funk just kept coming out, finally i pulled the pipe plug out of the block on the passenger side and i had to hammer a screw driver through this black sandy chunk of crap. i flushed water through every hole i could and it seems to be good now. check it out, could be your problem.