Also, I don't know if you can "fix" it with a different bootloader. I use different brownout detection level fuses than arduino also (need to sort out ISP programming to do that) as well as locking out runtime flash modifications. Really isp isn't that big of a hardship, less than $15 for nice parts (or a hacked up parallel cable) that only those folks who want to mess with reprogramming have to bear, and it means those folks a self-reliant and have more options (i.e. fuses and other "real" programmer goodies like more ROM and stability).
Arduino Forum - under 1k bootloader, any interest?
"My understanding from the datasheet... When the voltage level is too low, the processor will continue to run but may not correctly execute instructions. "