1994 Chevy S-10 runs good half the time

i have a1994 s-10 blazer with a 4.3 vortec.it will run good for a period of time then it will run like junk. it will run like the plugs are fouled out.and the exhaust is black smoke that has a sulfer smell.i replaced the egr valve about a year ago. could the egr be bad? and sometimes the check engine light comes on for a time but will go out after car sits for awhile.passed emissions test when running good. thank you jeff lamott