Hey!
Cheers for getting in touch. I just brought up the code and had a look - it seems the script is currently programmed to chose a random rock from your selection, and mine it. It does this because people asked for more randomisation - also, for small collections of rocks (e.g 3-iron spot), if just looking at the closest, it would repeatedly try and mine the rocks in a specific, repetitive order.
There's not much I can do about this other than suggesting you only mine in a smaller region. Equally, I could put an option in the UI but I would have to phrase it in such a way that is user friendly & explains what the option does in just a few words (this option would switch between picking rocks at random / picking closest). Maybe!
As for the low CPU thing, anything can happen... don't use that mode. It will screw up every script and there's nothing us scripters can do about it!
-Apa