I'm having trouble getting a stand-alone 328P device working at 3.3V.
I checked the brownout detection fuse bits, and they are encoded to 101b (max brownout of 2.9V).
I have a 10 MHz resonator hooked up to the device, well within the Freq/Voltage curve in the datasheet. Please not that IT WORKS FINE at 5V. I created a new entry in boards.txt and specified 10MHz, and it when I program the device on the Uno board, pop it out, and drop it into my protoboard, it works perfect at 5V. My test program is a blink on pin 0 at 1sec period. Works like a charm at 5V.
When I drop the supply down to 3.3V, it doesn't toggle the output at all (it is hooked to a scope). The resonator crystal is oscillating at 10MHz, but no output changes.
I'm definitely supplying >3.2V according to my scope, so it shouldn't be triggering brownout according to the fuse settings that I can read when it is plugged into the UNO board (using avr/boot.h macros for fuse reads).
What am I missing? I get the feeling it is something obvious...