It could be almost anything. Since we don’t know, geographically, where you are it is difficult to determine if weather might be a factor.
Dropouts and interference can sometimes be caused by loose coaxial cable. Check your connection to the cable modem, and also to the wall. Tighten both as much as you can.
If you can log into your modem and look at the frequency/error grid (each modem does this a little differently, so you have to do some research. If you have channel 12 (for example, on the modem – not the TV) showing a lot of uncorrectable errors, that can contribute to slowness and dropout.
Weather makes a difference.
New electronics in the house (new microwave oven? neighbor’s ham radio kit?) can cause interference. Old style TVs (pre digital) were notorious for cable modem issues.
Bottom line: We need more information. What cable modem to you have? What part of the country are you in? Have you unplugged your modem and router recently?