I've put together my first BBB (a hand-me-down RevD2 from a friend) and everything is working... mostly. It powers on fine, and all the voltages are fine (~4.8 at all the places the instructions say to test). However, when I uploaded the Blink sketch to test it out, the LED on pin 13 blinks when it's supposed to be on. "That's weird" I thought, so I set the sketch to simply switch the LED on, and it still blinks. I broke out the multimeter and the voltage going through the LED is all over the shop, from ~2.7 up to ~6.8 volts and everywhere in between. It does hit 4.8 specifically occasionally, but varies wildly.
Now, I'm a total beginner with this stuff and don't really know where to go from here. There don't appear to be any problems with communicating with the chip or with connectivity at the board but obviously something is going wrong somewhere. Some of my solder joints aren't the prettiest but they're all generally "good" (shiny etc). I'm a soldering newbie too, is it possible I've overheated one of the components or something? My rudimentary understanding suggests that if all the voltages are ok at the power buses then it's probably not a faulty component, but what do I know

So, err, what do I do now to try and work out what's wrong? I'm not sure if this is even the best place to ask, but I don't know where else to go really, so any advice would be very much appreciated.
Cheers!
-Thom M