hi all i have a vp calais on duel fuel and last couple of days it's been backing out the exhaust and front backfiring as well it starts ok but get 2 a quarter and thats when it backfires its blown my pod off and back fired out exhaust leads and plugs was changed 6 months ago i also have done a diagnostic test and nothing was flashing it will stall before or after backfires both front and back how can i fix this problem??? any help will be good