The issue is either the output is oscillating or it's margin is too big, such as +-250mV for a 3.3V regulator. I discovered some thermal issues new arduino mega 2560 regulators a while back. Under 12V external supply, they heat up and eventually break down, making the device unresponsive. The heat is excessive and burns my finger. I did a calculator using a typical regulator. Their copper on the circuit board should dissipate enough for 12V input but this regulator is not. It's brand name is indicated by ƆII, or a backward C, with two parallel lines that each are broken in the middle. Does anyone recognize this company logo?
That was over a month ago. Now new issues, their new arduino wireless sd shield has the same line of regulators 3.3V version. ƆII 3mF 17-33. It produces an audible ringing or buzzing sound when I have both xbee module and micro sd card plugged in and most of the time that sound indicates the output is not well regulated and my stuff don't run. I know most of you probably don't get newest arduinos so much but in case some have the newest arduino boards, look at the regulator and test its stability. Someone else that bought micros tested the regulators. Results are bad:
The older arduino boards (say 6 months to 1 yr ago) have typical LDO regulators and their spec sheets are easy to find. They all work fine, with 5V version against heat produced when supplied with 12V with a current draw around 300mA, or with 3.3V version supporting both xbee wifi or digimesh and microsd card. Did arduino team cut corners or just got screwed by manufacturers/suppliers?