We have an older LP gas furnace and water heater. If the water heater burner is going and the furnace burner lights, the furnace burner will not stay lit. The furnace keeps cycling off and relighting. The pilot and burner shut off for a few seconds, then the pilot lights, then the burner lights and burns for a very short time, and then the cycle repeats itself.

When I turn the water heater control to "Pilot Only," then the furnace burner seems to run fine. Do we have a gas valve problem? If so, is it in the furnace, or is it the one on the outside of the house where the gas line comes into the house? Please help. We appreciate your help.