I don't know what they use in the reissue Muffs. The vintage ones had germanium transistors that were notoriously inconsistent. The characteristics varied so much, that you had to try individual pedals to be sure you got a good one, and even then, the sound would change, depending on the kind of battery, how much charge it had left, and the temperature of the air in the room.

The reissue models are probably built with silicon transistors, which are much more reliable and consistent. If it is suddenly misbehaving, then something has changed. If you're good with a soldering iron, I suggest you look under the hood, check for loose or broken wires, and re-solder any connections on the circuit board that don't look nice and shiny.