I just had a hair pulling morning finding the source of a flec problem. I filled an order for 6 24x36 posters today and ended up printing an extra 4 because these little rectangular flecs were randomly appearing on some prints, and leaving white uninked spots. Long story short I found the source to be packed dust in the rubber teeth tracks of the carriage belt. It was randomly falling off from vibration and landing on the paper below! I vacuumed everything out then had to wipe the track well with alcohol and paper towel to finally clean it all off.