I was using a 3.3V pressure sensor with the 328p mini pro and thought it was an ideal solution until I had erratic results. It is overclocking as you say, commonly used on the pc. My Chinese mini clone has a 3.3 or 5V option so I assume I should use the 5 V rail and use a LLC to step it down. This kind of begs the question as to why a LLC is not built onto a '3.3V' board in the first place or am I missing something again?
I am still not clear though what happens to the crystal / frequency as you reduce the voltage below the threshold. I would have thought that there would have been a distinct cut off voltage and it would stop oscillating but obviously not. I suppose it has something to do with the rise time to resonance.
NICK Off topic. What logic analyzer do you use and perhaps recommend (cheap if possible)for this kind of work as I see many "logic" pictures on your site which help to see what is going on.