Grant,


I would like to elaborate on this.

When the aluminium head over-heats, it expands severely.
The head bolts, as a result, suffer from plastic stretching deformation.

I.e. the bolts actually become longer than they originally were.
They don't loosen by turning out, but after the engine cools back down, they no longer hold the head down as tightly.
If the bolts don't stretch, it may even be possible for the head bolt threads in the cylinder housing to be pulled out.