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I would make sure the part cooling fan is on full speed while printing PLA (after the first layer if your software will allow it). I haven't had the strings or lines issue so much with PLA, but I have with PETG. The remedy for PETG was to first make sure my extruder was calibrated properly, and then adjust the retraction after I knew for sure that the extruder speed was correct. There's a Tom's Guide for calibrating the extruder.



Other possible causes of the strings would be printing at too high a temperature, printing at the wrong speed, and having a partial blockage in the nozzle. Start with the easy things first--lower your print temp a couple degrees and see if the situation improves. If you get to 5 degrees below recommended print temp and you are still having the issue, then go back to the normal print temp and try changing the print speed up/down and see what effects you get.

Oh, and it is really important to take into account the actual measured diameter of the filament and have this value set as the filament size/diameter in whatever software you use. This will impact the amount that is actually being extruded through the nozzle. Most of my PLA actually measures 1.71mm or so--not 1.75mm. Any inexpensive (preferably digital) caliper will get you the measurement you need. Having the correct filament size can magically clear up several problems that may occur.