Ask Smithy Xtra Idle Fairmont

My wife has a 2003 BA Fairmont on dual-fuel running a vapor injection system. Lately she has noticed it had developed a number of problems. It sometimes idles roughly and wants to stall, there seems to be a rattle/vibration coming from the engine when cold, and there?s a popping sound when it?s running on LPG. The car has done approx 130,000 km, and approx. 40,000 km since the LPG conversion three years ago. Any thoughts on what the problem might be?