I work as a A&P for a small cargo company. I worked nights for 20 years and we used to have well over 100 piston twins that flew in all weather mostly at night. I have much experience fixing both Janitrols and Southwind heaters. The first problem is that a majority of mechanics can’t read a simple wiring diagram well enough to help with the troubleshooting process. So, they just throw parts at it hope to get lucky. Second, few mechanical things function the same at -20F as they do at 45 or 50F, which can sometimes make troubleshooting a challenge. Cessna 310Rs had a Southwind heater ( I don’t remember which model). The combustion air pressure switch is actuated by a cam attached to a little door that swings open when the combustion air blower is operating. If the little door gets dirty or the pivot points get worn or the blower turns a little too slow, the switch won’t close and the heater won’t work. If the heater works initially then quits above 5000’, I’d suspect a fuel delivery problem. The fuel pattern in the burner can needs to be a nice cone shape. The fuel nozzle being partially restricted or the pressure being low will cause the pattern to be off. That will cause the fire to go out in more difficult operating environments such as higher altitudes or in a decent when more air is being force into the burner can by ram air acting on that little forward facing scoop on the right lower side of the nose.