Several people have suggested that the temperature at the entry of the barrel is very important to printing with PLA.  This makes sense.  If the temperature at the barrel entry is above the melting point for PLA, it can melt right there and ooze down around the threads.

Maybe I should assembly my extruder with just the thermistor on the barrel entry, and heat it until it is just below the melting point of PLA – then check the temperature at the nozzle.

Or, I could get a second thermistor and use it at the top of the barrel while keeping the original thermistor at the nozzle.  I could only use one set at a time (since I don’t know how to hack the electronics/extruder board), but that shouldn’t be an issue.