Hard drive isn't the issue at that speed. 1.6 meg (I'm assuming here you mean Mbps, megabits per second, not MB/s, which is megabytes, which is 8 times larger/faster) is like 200kB/s, your laptop hard drive is faster than that unless it's from 1985, in which case, how the heck did you even manage to get on this website?
if it varies greatly based on time of day (and not based on what you're watching/doing at home) then it's the cable company having issues and short of rebooting your cable modem there isn't much you can do without their help.