1990 GMC low rpm surge

I have a 1990 GMC K1500 that surges at low rpm (whether at "traffic jam" speed or in overdrive). I removed the vacuum line from the EGR valve and plugged it to find the surge is cured, but the valves rattle terribly bad (I have to burn 93 octane to get them to quit pinging). Can anyone out ther give me a clue to my problem? I have tried several mechanics in my area. Thanks.