slow internet

For some reason my internet has been running extremely slow, for example if i was to watch a short clip online it takes a really long time to buffer, on youtube, usually when i watched a clip the video would just stream, but now i have to wait for the clip to be buffered at about 1/2 way so that by the time i get to their my internet will keep up speed. Im using a cable modem and i have noticed that downloads are downloading at about 56/78kbs but im using cable......how do i fix this?