I constantly get black bits on my washed clothes?

I have a Hoover Nextra machine (model no. HNF315780) which is cold fill only and contantly get little black bits all over my washing. It has been suggested that I do a very hot wash with no clothes in it about once a month but this only cures the problem for one or two washes. Most of my laundry is done at 40 degrees sometimes 60 degrees. My machine is 4 years old but I have had this problem for some time