Why is my car backfiring?

[Patti2912]

Hey guys,
I have been having a bit of trouble with my car lately in regards to mainly the duel fuel system.
I had a full service the other day, and got a lot of parts fixed/replaced, and my baby is now running beautiful, and not missing a beat at all,
Except when i switch it over to gas. It runs perfectly, but every now and then on acceleration it back fires. This happened once prior to the service and blew the top off the gas mixer which needed to be replaced so im not wanting to do that again.
To give you guys an understanding, it sort of feels as though something builds up prior to backfiring, as i feel a slight decrease in acceleration power which happens gradually over 10 - 15 minutes maybe, then it pops, and the full acceleration power is restored.
Can anyone suggest what this may be or what i should be looking at to determine the problem and get it fixed?

Cheers.

-UPDATE-
Have noticed that the throddle response is delayed whilst driving with gas, is this normal or is something ****ed?
If its common then that may explain the above issue as i am driving it as if it has instand throddle response like when on petrol. I don't over do it when its on gas as i realize the power loss and i only drive it on gas whilst cruising, but this morning i drove like a grandma and had no issue.
Think i just answered my own question or is it still something mechanical?