What can cause a hot CHT?

This has now happened to me several times: one of my CHT temps will
bounce around between normal and off-the-scale hot, and sometimes will
cut out entirely. All other indications (EGTs, the other CHTS, oil
temp, airspeed etc.) are normal.

This has happened before and replacing the CHT probe fixed the problem.

Trick is, 1) it's happening a lot (thank god the thing is still under
warranty or I'd be spending of money on probes) and 2) this only happens
when the engine is at full power. If it's close to idle the CHT reads
normal. This makes me think that it is NOT (just) a bad probe. On the
other hand, it seems physically impossible for the readings to be real
because it seems physically impossible for a cylinder head temp to rise
and fall that fast (+/- 200 degrees in a second or two).

What could be causing this? Is it potentially hazardous?

Thanks,
rg