Tight valves could be a problem. However, you say the fuel lines are frosting. That is
an indication of a problem in the LPG fuel system. It sounds like the liquid LPG is not being converted to a vapor state so the engine can use it for fuel. I suggest you check the following:
1- Coolant level in radiator. Low coolant can cause the LPG vaporizer to frost over and restrict or stop the flow of fuel to the engine.
2- Check for a clear flow of coolant thru the vaporizer. On older units a lot of things can go wrong, a hose liner could come loose and stop the flow, scale buildup could stop the flow, a weak water pump could the the problem, etc.
3- Replace the vaporizer. A bad vaporizer can also freeze and prevent the engine from getting the fuel in the proper form.
These are things I've seen in the past that can cause the probelm you describe.
Hope this helps.