I completed my PrintrBot build on 21-Nov-2012, and I've been printing things with reasonable success up until now (approaching 2kg total filament used). My heated bed is installed on top of 8mm of Neoprene foam, and the foam is on top of the birch table. I have some metallic tape on the top surface of the neoprene to distribute heat and reduce oxidation of the foam. On the top side of the bed I have a 1/8" sheet of glass. I usually warm the bed up to an indicated temperature of 90C before I start printing, but I have it set to 81C (indicated) while printing I have not ever attempted to measure the temperature of the top of the glass.
I had noticed instances where the printed bed temperature would fall off while printing... sometimes down to 69C or lower... other times it would hold 81C without trouble. I had attributed this to a software problem and hadn't pursued it. Yesterday while printing I noticed that my bed temperature dropped off all the way to 29C. I didn't have anything else to print, so I shut down when I was done and didn't mess with it. Today I went to print something else, and the bed wouldn't warm up significantly. I got it to rise from 18C to 18.67C, then eventually 19.4C. I have the printrbot neatly mounted in a wooden box, so I loathe pulling it out and unsnipping all the wire ties, etc., to service it...but I went ahead and did it. What I found was that the heated bed connector at the Printrbot main board was charred and blackened around each of the 4 pins

. I disconnected the bed and gave a warmup command, and did not find any voltage at the pins

. I don't usually trust a single measurement, though, and after four or five power cycles and re-measurements I started reading 12V at the pins (so either I did a crappy job measuring at first or the pins came back to life).

I'm considering installing an external relay in the heated bed circuit so that the PB mainboard provides only a switched voltage and the board is directly powered by the PSU.
I'm also wondering whether a higher-power PSU would help (after reading the above) but that seems like a fix to a problem other than the immediate one.
Just thought I'd mention it in case someone else is having a similar set of symptoms.