I am new to this forum and have a similar problem so wondered if you ever got to the root of this problem? My board is small and no heavy currents at all BUT the Atmega328 I am using can sometimes hang in this strange state. Resetting does nothing, the only way to get it back is disconnecting the battery and start it up again. When it hangs, the clock signals become smaller (1.5v ish) even though the supply is 3.3v. I wondered if anyone knows what state the 328 is in when it does this? I have lots of smoothing caps close to the 328 and there should be no currents above 15mA anywhere on the board. Thanks in advance for any suggestions.