+1 on sanding the thermocouple. Its job is to sense the main burner, if it is dirty/damaged, it won't signal the main valve to open.
Also, how is the gas supply line coming to the unit? Often, air will get into the system and can be a SOB to purge. Until it is purged, it will work like you describe. Run, then die and not refire.
I've also had spider webs up in the burners in units that sat unused for a spell. Get some compressed air and clean out the cobwebs.
Keep us posted.