If it's doing this on LPG is the tank valve opened FULLY and is the tank oriented in the correct position so the draw tube in it is at the bottom, had a simislar problem a while back,operator was just "cracking" the valve open a little and they were not putting the tank in the right position to get full use of its capacity, also had one customer who was running a truck off of a barbecue grill tank and wondering why it wasnt running right