The two stage gas valve (White Rodgers 36E54-201) in a Trane XV90 LP gas furnace is giving me fits. The first stage LP gas pressure is supposed to be adjusted to between 4.0 and 4.5 inches of W.C. The problem is that the gas valve doesn't maintain this setting but drops lower and so sometime during the night, the furnace won't stay lit due to the low gas pressure. The inlet LP gas pressure is 12 inches of W.C. I suspect there is a mechanical problem with the first stage adjustment mechanism because the adjustment is made with a hex key wrench and as I am pulling the hex key out of the adjustment screw, the manometer reading changes and I have to try to adjust again. It's like the adjustment screw is stripped out of its mechanism in the valve. Has anyone seen this before? Is there a way to fix it or does the valve have to be replaced? Thanks.