Sounds like lack of fuel. Is the regulator frosting up? if yes that indicates poor or no coolant flow. If your lift has a vacuum lock-off (VFF30) check the vac hose to the lock-off valve or the filter may be plugged inside. If you have a electric lock-off confirm it works (clicks) or try running the lift without it.
Also if the lock-off is electric, you may have a propane filter inside the bulkhead fitting/housing attached to the frame or below the tank.
Tank fittings damaged hoses can also cause fuel flow loss.
If you have the woodward fuel system the mixer diaphragm may have failed.