2004 GMC sierra Denali 6.0 po300 codes

Question

My 04 sierra 6.0 has an intermitant missfire code at speeds above 60 MPH. The check engine light goes out after driving at lower speeds. I did replace the plug wires and plugs.

Answer 1

Misfire condition above 60 MPH, the only cause for above 60 MPH  misfire condition is a partial plugged exhaust system. These engine are sensitive, if allowed to remain too long what happens is it develops a crack between the intake valve and exhaust valve. Then you would end up with a misfire at idle.