the vff30 is one clue, and age of cranking battery is another. any impco (pre-tier1 CARB) system needs to crank pretty fast, to draw the fuel in, and using the vff30 lockoff (instead of an electrical solenoid activated lockoff) makes that requirement even greater. put NEW batteries in these trucks and see how they do.
And don't touch the throttle, and do have the the idle speed where it should be, do have the tune up timing and spark plug gap to be correct for LPG machines (which should be different than for a gasoline motor).
I am amazed at how often this complaint is related to spark plug gap.