First thing you want to do is figure out what temperature your hot end is actually at. To do that, buy an inexpensive point and shoot infrared thermometer similar to one of these: http://www.amazon.com/s/ref=nb_sb_noss_1/190-9732736-0069215?url=search-alias%3Daps&field-keywords=infrared%20thermometer&sprefix=infra%2Caps&rh=i%3Aaps%2Ck%3Ainfrared%20thermometer
You should be able to find one locally at an auto repair store or general hardware store. The Fluke ones are the top of the line, but spendy. The others should all work for this purpose.
Point and see what temperature you are getting, and compare that to what your printer thinks it is set at. If there is a significant difference then you start investigating the other bits.
If your printer is set correctly to 210-220c-ish for PLA and it’s still doing that, and the infrared thermometer indicates you are at the correct temperature, then maybe you have an odd batch of PLA that melts at lower temperatures, or you are printing too thin layers and getting a bunch of backpressure at the end. if you ordered your filliament from someplace that might be less than ideal, try ordering another roll from lulzbot or one of the other known good distributers.