xantia having some hard time egniting

[erez]

Hi,
My xantia is 2.0i 8V 1997, (green) from israel?

The engine is having hard time egniting the fuel, it's more evident when using LPG,
I already replaced the spark plugs twice this week, changed the spark plugs cabels, what else should i check? what else is there to replace?
today there was also a small backfire around 1000 rpm.
Thanx, Erez.