I have an Anet A8 with a MK8 extruder. Recently I've noticed it's making the skipping sound during print which manifests in missing parts of random layers. It's the same skipping sound as when the extruder is too close to the bed and can't force the filament out.

I always print with PLA, never changed from 200c.

Things I've tried:-

1. disabling the hotend fan.
2. placing a washer above the lower bolt for the spring to increase spring strength.
3. moved the brass coloured wheel into the center via it's two grub screws.

Any other ideas?