Lights

Does it have a set of built-in rechargeable batteries?

If so, your batteries are dead... no more recharge cycles left in them.

The fridge trick is used in prisons to prolong the life of batteries. It helps restore some of the charge capacity after it's been run down.

Biting batteries achieves the same... but shortens the life dramatically in a different way.

Basically... the super-cheap bit resulted in them shipping super-cheap batteries.

It's definitely the head unit and not the battery, I've placed both in the freezer, one-at-a-time and only have problems when the torch itself is cold.

Seems as though low-power does not generate enough warmth to stop it malfunctioning either. grrr.