Voltage is part of the problem for sure, which a variable transformer can definitely help keep voltage at a steady operation (you may not even need that since i

Voltage is part of the problem for sure, which a variable transformer can definitely help keep voltage at a steady operation (you may not even need that since it's a Thermoelectric generator... I'm not familiar enough). But it's primarily wattage - at some point the overall power dips too low to maintain voltage at that amperage to keep the system alive.