1998 Toyota Camry Ignition System

I have a Toyota camry 1998 4 cylinder car which has done approx. 150,000 Km, at approx. 140,000 Km I got the car converted to run on LPG as well.
It ran well for approx. 12 months approx 12,000 Km and then started backfiring when accelerating so I got the car checked by the company who converted the car to LPG. They readjusted the GAS mix, changed the air filter and the spark plugs It ran for approx. 1 week well and again started backfiring again. If I change it to Petrol it runs better and does not backfire as much.
One peculiar thing is on a round about (right hand drive car) when running on gas when turning it starts backfiring almost every time.
The sparks at the moment are NGK (standard ones) not Platinum.
Please recommend what should be checked.