Well, if all of the machines are failing, it's a common component. Router, modem, or the ISP.
Some things you can try here for the modem/router.
- Update the firmware to the latest version available on the manufacturer's site.
- Reset the router to factory defaults and reconfigure.
Many times these measures will resolve a lot of intermittent issues.
It is possibly a heat issue, try directing a small fan directly on the modem/router and see if that helps.