I would not blame the LPG tank or LPG flow, as this would be worse at higher speed, rather than lower speed, kind of like when you run out of gas.
I would make you pull the plugs and gap them, if you worked for me.
if firing was off you might not notice at higher speeds. same with poor compression on only 1 cylinder. it might feel like a bad spark plug wire would. which has a tendency to "smooth out" when not under heavy load.