With your cable speed varying like that it's one of three things; 1.) A server issue at the local head end; 2.) A return plant problem, probably ingress; 3.) node loading. However, during the day, unless you're in an area where it's heavy usage, it should be pretty quick. I'd probably suggest that you call the cable company and complain about the speed issues. Also, you can get into
Your Cable Modem(typically) and see if the cable modem is fluctuating. Look at signal to noise ratio, upstream power and downstream power. Upstream should be less than 55 dB, downstream power should be greater than -15 dB (-10dB and up is better) and signal to noise should be greater than 33 dB