Key may be "certain times of the day" - e.g. afternoon.
Agree with NerdyComputerGuy.
In addition, I would check all of the incoming ISP connections, cables, splitters, etc. for signs of degradation and/or damage. Splitters can be especially problematic and even more so if outside in the weather.
For example if the problem occurs in the afternoon when the temperatures may be warmer or direct sun is hitting the cables and connection paths - expansion may cause problems.
Still the specific location could be anywhere.
Sometimes just disconnecting, cleaning, and reconnecting any coax connections will solve such intermittent problems.
Hopefully your ISP will provide a new router and run some connectivity testing from their end as...