I would have expected most all of the trucks with the "MTRTWR8" connection concern to have been fixed long ago, this PSB came out in 2004 or 2005 and only was concerned with 4g64 and 4g63 motors produced in the 1st 1/2 of 2004, and really did keep the motor from charging properly, so much so that correction was done asap.
Since we still don't have a serial number series to be sure,,,
Some other things his symptoms call to mind would be not enough fuel getting to or through the LPG regulator (dirty filter, or bad "rego" quick disconnect) or possibly an "almost will fit" rotor button or bad distributor shaft bearing that has a little play that shows up at higher rpm.
Whitebuzzbait, I am pretty sure that no tb45 (a straight 6 cylinder) ever got stuffed into a GC25k or GP25k (they all used the 4g6* series motors).