Only 2 things come to mind here:
1) Bad material qualify (can happen if you store the filament without a sealed container in an especially moist environment)
2) You extruder motor is intermittently skipping steps
Potential things to try:
1) Try with other materials to see if it’s a material specific issue.
2) Wipe the first 1ft of the filament with a thin layer of Olive oil before reloading it in the machine.
3) Clean out your extruder gears and nozzle.