Why does my ignition coil malfunctioning?

It is the 3th time that I have changed the coil...

What can be the problem?

 

HI, can you give little bit more information? (year, make, model, etc.)

 

Sorry,

I have this problem now about a month..

My car is converted to LPG but the problem is also when I drive on gasolene.

So:

Stationair the RPM is not constant. It is moving from max 1000 rpm to almost 500Rpm. If it hits the 500 it looks like that the motor stops.

If I then start driving the car is stuttering and around the 1500RPM the engine light is coming up.
After I have checked the fault with ELM5(OBDwiz) it says everey time P306 (misfire cilinder #6)
I have replaced the ignition coil several times but the problems comes back.

What can this be? (sorry for my bad English)