Screenplay

I am having problems with my speed. I have cox cable advertised at 4.0. When I do speed test some days I will be up to 5.1 but on others I will be down to .8. What is the deal??? I always keep up with ad aware and virus scans. Plus I have already downloaded the cablenut program (think that is why I get 5.1 sometimes). Any help would be great. P.S. could it be my modem? It is old Surfboard sb3100