Yeah, all is well on the day with fresh new batteries
I suppose the thing to do here is just run a dying batteries test.
Oooh, scary! :O I'm just wrapping my head around 8 MHz. Is there even a boards configuration for lower?
The input can be anything from 1.0 to 5.5V, so this can also be used with 2x or 3x AA battery packs (even 4, if NiMh), and it will continue to work when hooked up to a 5V power supply. The regulator will work down to less than 0.5V and can squeeze the very last bit of energy from the battery.
ST makes a booster - L6920 - that is about the best I've found in terms of quiescent power consumption (down to some 20uA or less depending on input voltage) and low voltage startup behavior. This will run down to 0.5V or so (actually, 0.4xx V in practice if you're not drawing much current). Not sure how low in cost you want to go; there are definitely cheaper ones out there if you don't mind higher idle current.
Do you have any plans or schematics online? I'm also working on a very low-powered Arduino variant geared toward energy harvesting projects; might be good to compare notes.