If-when the engine is cold-the engine will idle fairly good, but starts to sputter when the gas pedal is depressed & the symptoms go away once the engine warms up, it's a symptom of a tarred up regulator. My advice would be to replace the regulator & shut off valve, clean the mixer & replace the mixer diaphragm.
On a side note- you will experience a decrease in performance from a LP fired engine when the ambient temps are extremely low. The only fuel pressure the fuel system gets is from the pressure in the propane tank, which drops dramatically once the temp drops.
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