Since Steam servers can be inundated at times it may not be something you can control. Since the download will eventually arrive, there is nothing to fix unless you feel Steam needs to get more internet pipes.
I've been having a problem with Steam downloads they have been very erratic and dropping very low, it should be hanging around 600-700 KBS but it'll drop all the way down to below 100 KBS and back up again the download speeds are unstable, I have no other downloads running and I have Steam trusted through my firewall and Norton. This is a Steam specific issue all other downloads are fine and stay steady at 650-700, but Steam is highly erratic and I have no idea why. Would someone please help me? I've attached screenshots of it's download performance.