back firing

[67cst]

hey guys, the 71 that i got 2 weeks ago is running good but i notice when im climbing a hill or giving it a fair bit of gas it back fires a bit, not loud at all, more like a pop, it seems like its miss firing almost, what is causing this? should i put some new plugs and wires in? would it be the propane maybe...ive heard when its cold propane can act weird...any suggestions would be great

thanks, Derek