multiple misfire

hi there !!! i have a very complex problem with my Sierra 3500. i have a 2001 GMC Sierra 3500 lt ,*dual wheel , automatic , V8 ,6.0 , Gas , 320,000 original engine . I keep getting a "service engine soon " blinking light after 2000 RPM. I ran the scanner i got the P0300 code "random or multiple...