Whats wrong with my 1971 Plymouth Duster?

The engine won't turn over like it normally does and then it will only stay going at a higher RPM. THe engine "stalled" and turned off two times while i was driving. I took off the fuel filter i had and cleaned out a bunch of nasty looking sludge. thought it would solve the problem but it didn't. Could it be the fuel pump? or maybe a bad wire/spark plug(s)?? HELP!!!!!