My problem was cured with Robgrea's solution to put a fan on the Extruder motor. There are many possible causes of this problem, so you do need to start with a perfectly calibrated bed to rule out those issues but if ABS prints fine, this may well be the answer to some folks problems since PLA has a lower melting temperature.
On my printer, ABS prints perfectly. The bed is calibrated and produces excellent quality prints, but when I started trying to print PLA, I could be 4 hours into a print and the extruder would stop feeding.
After watching a print for several hours, I noticed the clicking start. A little at first and then increasing till finally the print failed.
I have Repetier Host software and Adruino firmware installed in my XYZprinter 1.0 and I tried various heat settings and speed combinations, but could not control the missteps of the extruder until I put a fan on it.
I adjusted the extruder temperature to 225 degrees to compensate for the fan blowing on the extruder and now my PLA prints work great! I am printing at .3mm layer height with the same settings that my ABS uses.
Touch any of the steppers and you will find they are very hot. With the fan blowing on the extruder stepper, I can now touch the stepper without burning my finger.
I don't think I would have figured this out on my own, so I want to personally thank Robgrea for posting his solution.
My previous experience with CNC machines has shown that stepper motors do run very warm to hot, but given that XYZ put a caution label on the extruder stepper indicates to me that they purposely set the current to be high enough to feed filament properly and that may be why they did not have it setup for PLA in the first place.