It sounds like you might have some of the PLA burning up in the nozzle and/or on the surface of the nozzle. This can sometimes be caused by over extrusion or impurities in the material burning up.
I have seen this a lot in Zortrax Z-ABS when using the colors Yellow, White, Blue, and Green. If I remember correctly it was just the color filler burning up at the temps they had set but this was a specific thing to Zortrax and their material.
What PLA are you running and how hot are we running on it?