You said, "...i did run for 2 weeks on proane without issues." so I have to assume the roll-out is due to the new installation. Unfortunately you'll need to follow the conversion kit directions because they give you the details of what pressures you should see at the gas valve input and at the gas valve output to the burners. Also verify with the furnace and gas valve manufacturer that they can be used with NG. It is curious that it takes 15 to 20 minutes before roll-out. How does the flame * Is it strong or does it appear to wave like a breeze is flowing from * You need to watch when it gets close to flame roll-out and possibly see what is the cause.
I'm sorry I have no better suggestions. Please keep us posted.