Sticky valve guide. Has the truck been idle for an extended period of time and then run without a thorough flush or replacement of the gas tank? Stale gas forms varnish, which will accumulate in the valve guides and cause valves to stick on the SECOND engine start. The first one will appear to be good, then once the cruddy gas cools in the guides, the sticking begins. It's a very common problem when waking up an engine after long storage without dealing with the gum and varnish in the gas tank.
If that's the case, it's a roll of the dice whether anything short of a disassembly of the head and reaming or cleaning the guides will help. External guide cleaning with solvent and/or gas additives are seldom effective.
For a definitive check of the pushrod find a piece of glass the right width so the flared ends of the rod hang over the edge, and roll it. A straight pushrod will not show a gap anywhere in the middle as it rolls across the glass. Stovebolt pushrods have approximately the rigidity of half-cooked spaghetti, so they're pretty easy to straighten if you're careful.
Jerry
