[willowst203]

Why is it that a burning log will seem to have burnt out (become red/orange with no flame) and then suddenly light again? 

[Borek]

Hard to tell what you mean, as the description is ambiguous. If I were to guess what you mean - the flame can appear only when there is enough gaseous combustible products. These get produced by the hot log, and get depleted while burning. When their amount gets too low flame disappears, it shows back when the amount gases builds up again. Basically system oscillates between two states.