1998 GMC 3500 pings between 55 and 60

1998 GMC 3500 mileage: 50,000. I am having a pinging problem while driving down the highway at 55 to 60 mph, flat road and sometimes when I accelerate to pass another vehicle. When starting out whether the engine is warm or cold driving back on the freeway it's as if there is lack of power, sputtering as if it just doesn't want to go and then after a minute it will start to speed up fine. This is an intermittent problem as it doesn't always happen. I have taken it to the dealer twice. The first time they changed the fuel pressure regulator. They dead headed the fuel pump, reading was 70 psi. The engine has never set a code so they are also at a loss. Sometimes it does it sometimes not. The engine seemed to work for a few hours after they changed the fuel regulator, but on a trip the problem returned again. I have tried different levels of octane fuel, changed fuel filters, air intake is fine. When it starts to ping it will do it when the engine reads cool or warm, I just climbed through Mt. Shasta and had no problem with pinging, though sometimes when I tried to pass it once again seemed to lack power and then just take off and run fine. I'm at a loss and the dealer has no ideas. Any suggestions on what I could trouble shoot to solve this. Dealers answer has been to drive it till it gets worse, which will probably happen when I'm on a trip. Thanks for any help.