Marlin won't boot unless serial monitor is open

Using a Mega 2560 and Marlin 1.1-RC6 (I've got a CoreXY setup put together here), I'm having trouble getting Marlin to boot up. When I connect power, I get a sort of flickering screen (I'm using a "RepRapDiscount Smart Controller" clone with the character-based LCD). If I hit the reset button on the RAMPS, it starts flickering with a bit of the Marlin boot screen (the top left corner of the "bubble", 1-3 characters of it), and then goes out. On and off, on and off.

If I flash the firmware via the Arduino IDE, after flashing, Marlin boots normally after the flash is complete.

Also, and I just figured this out, if I have the Arduino IDE "serial monitor" open, it boots fine. Strange.

I assume I've screwed something up, but I can't figure out what. Anyone have any ideas?

Sounds like damaged voltage regulator

When the mega is plugged into the ramps its generates 5v from the 12v fed into the vin pin of the mega. This goes threw a voltage regulator to produce 5v.

Now when you plug in the USB cable the 5v is powered from USB, ignoring the voltage regulator

I disconnected the 5v regulator on my board (because it was damaged). I plan to power the Arduino via the USB, and the RAMPS with the 12v supply. I'm not confident that this is the issue, however, because it behaved this way before I killed the 5v regulator.

Also, to be totally clear here, I'm not trying to boot it without powering via USB; If I plug in the USB, it doesn't boot, until such time that either A) I reflash the firmware, or I open the Arduino IDE's serial monitor. Once I do either of those things, it boots fine.