Well, the nasty question is which 3 of the 6 boards are failing, yours or the assemblers
Can you check the crystal to see if its oscillating? Maybe you have the wrong load caps on this time around?
Are the voltage levels right? Can you probe them with an oscilloscope to check for signal quality? (though I'd spend more time on the oscillator question)
Are there any tantalum caps in your design (and if so, are they backwards)?