Well especially since you changed the thermistor you really dont know where you are temp wise. I have a digital temp meter with a thermocouple probe. I take some kapton and tape the thermocouple to the side of the nozzle itself. Its the only way really. You would be surprised how far off it can be.

For pla i cant say that i have heard of it ever printng as high as 220. Some people with certain machines may have to put it that high but if you took a true temp reading of the nozzle you would find that its more in the 190 range and setting high is just to compensate for the temp being off. To get a better idea lower your temp to 180 and extrude some plastic. If it wont extrude then raise the temp 5 deg and try again and so on. You want to find the point where the plasic just extrudes out the nozzle with a slight amt of difficulty but without the drive gear stripping out on the filament. Once you find that temp setting, raise the temp 10 deg and you should be close to the proper temp for the filament.