The 350 in my Monte is popping on the highway I dont get it around town .Only get it at steady highway speed. Took a look at the plugs and the number 5 is pretty white much whiter than all the rest. I thought it might be ignition so I bypassed the MSD box using a balast resistor for my Mallory dist. got a new coil and new wires and no change. I ran the car with the valve cover off in the driveway and the #5 rocker moves the same as the rest-at idle but again this isnt where the problem is. Should I pull the manifold and change the lifter ? Is there something else I can check? .I cant reproduce the problem with out being on the highway. Thanks for the help.