it sounds like a excessive draft issue check your manual for restrictor settings
Sure does ! Glad a protector of sorts cures the airflow issue around the pilot to rule out mechanical or electrical problems. I'm sure you would like to leave it alone since it works, but........if it has too much draft it will be killing the efficiency allowing a lot more heat than necessary up the vent. They already eat enough gas ! I don't know if this unit has a restrictor plate or adjustment for draft, but that would be the cure. That's the small baffle piece, or maybe called a damper I thought might be missing. Manufacturers call them different things.
You mentioned it ran perfectly fine when it was set up for LP. Was this at another location or was it connected to the same exhaust vent? Same termination cap?
If you slow the draft down too much, the spillage switch is going to heat up and shut the burner and pilot down on safety.
So the draft has to be just right. That would be great if it's adjustable.
When you mentioned it was shutting down in your garage with no exhaust vent, it wouldn't have a draft to allow hot exhaust up the vent hole and the disc switch (spillage) should shut it down when it heated up.
Here's what happens when there is too much vent restriction, or poor draft usually from not enough chimney height. (another section of B-vent is added to cure) When the main burner lights, there is little to no draft in the cold chimney and the heat doesn't go up the exhaust quick enough to start drafting. Some of this exhaust and heat spills out the air intake of the exhaust diverter, and trips the spillage switch
before the chimney gets drafting. In that case the main burner will only stay lit long enough to trip the switch. Some switches have a reset button on them, others are auto reset. So if you do something to slow the flow too much, that is what will happen. When you can get to the back easily, it's simple to light the main burner, and shake a match out and hold the smoking match up to the diverter intake to make sure it's pulling air in. If the smoke lingers at the intake, it's not drafting. Exhaust fans in the house and other factors can cause poor draft or back draft. The spillage switch is there to stop exhaust from entering the room.