I never got back to this, had to go to other customers.
But the tech that did found a bad wire/connection (not spark plug wire) at the coils.
Thanks for the help, it led us in the right direction.

Thanks guys.
I didn't get back to this today.
I'll let you know what happens.

Is this a newer truck with 4 ignition coils? If so, the wires break right at the plug that goes into each coil.

Sounds to me like the coil is breaking down when it gets warm/hot

swoop223 has a good point- use good (not aftermarket) igniton wires & spark plugs ( I have good luck with A/C or Autolite plugs & NAPA make your own wires) The SMH stuff never seems to hold up.

If the ignition checks out, I'd then check the fuel system. Does the engine only run rough at idle? When was the last time the Impco regulator was changed? Is it a model J? What type of propane shut off valve does it have- vacuum VFF-30?

I have found that usually when the impco systems go bad they either are hard to start & won't accelerate when cold ( buildup of tar) or they begin to run rich when warm (primary port of regulator starts to leak) or VFF-30 vacuum line starts to feed engine LPG.

To check the VFF-30- pull the vacuum line & cover the vacuum port. Engine should run a few minutes before stalling- listen to engine to see if engine smooths out. If engine runs same reattach vacuum lline, start engine & shut off propane bottle. Listen to see if engine smooths out right before it stalls. If it does the regulator is bad.

Hope this helps

a couple of things come to mind
- you did use factory plugs not aftermarket ones right?
- have you checked the plug wires, make sure they aren't breaking down when they get hot. (usually they would start arcing against the engine head or anywhere they are close to the engine).

As for the fuel pressure? that sounds close to what it supposed to be, maybe slightly low. what is it doing when you accelerate? does it drop lower? or is it consistant?
if it drops then this could be the problem...
vaporizer/converter

good luck