I had a problem on a similar truck where the plugs weren't changed regularly and it caused some extensive backfiring. The Air filter was clogged horribly and when the backfire occured it transfered pressure to the regulator through the balance line damaging the seals in the regulator. I rebuilt the regulator and it took care of it. Obviously this isn't your exact cause but when they are overheated and poorly taken care of they will start to backfire like crazy.( I have A LOT of experience with these trucks).