What I've found with mine - though this may differ also - is that video decoding and encoding seem to be very "sensitive" tasks.
For example, while my OC'd N900's been 100% stable for weeks, it has always had two problems:
1. If I try to record a video without locking it to a frequency, the camera app would crash and
2. On bootup, the boot video would freeze after a couple seconds.
Everything else worked perfectly.

I just did some tests, and found that my 250mhz voltage was a little too low - I had it at 22. Bringing it up to 28 solved the problem entirely.

So, I'm thinking that between a couple of more sensitive tasks - and playing a video'd be good for at least one of them - we can get -close- to the needed value. Then, we add enough margin of error such that if its just a little below what's needed, it will still work right.

Speaking of that, if you guys have any good scriptable test ideas, I'd like to hear them.