Intermittent, regularly repeating network outages

On 7/1/2018, I upgraded our Comcast Business Internet service FROM Deluxe: 50/10 Mbps TO Deluxe 75/15 Mbps.

I retained the SMC D3G-CCR modem and the Static IP 13 block.

Pingdom is reporting intermittent, regularly repeating outages. For example:

Pingdom DOWN alert:

Uptime (www.farces.com) is down since 07/10/2018 05:22:25 PM.

Reason: Socket timeout, unable to connect to server

PingdomAlert UP:

Uptime (www.farces.com) is UP again at 07/10/2018 05:37:25 PM, after 15m of downtime.

Pingdom DOWN alert:

Uptime (www.farces.com) is down since 07/17/2018 05:12:25 PM.

Reason: Socket timeout, unable to connect to server

PingdomAlert UP:

Uptime (www.farces.com) is UP again at 07/17/2018 05:47:25 PM, after 35m of downtime.

Pingdom DOWN alert:

Uptime (www.farces.com) is down since 09/08/2018 12:07:25 PM.

Reason: Socket timeout, unable to connect to server

PingdomAlert UP:

Uptime (www.farces.com) is UP again at 09/08/2018 12:17:25 PM, after 10m of downtime.

Pingdom DOWN alert:

Uptime (www.farces.com) is down since 09/09/2018 05:47:25 PM.

Reason: Socket timeout, unable to connect to server

PingdomAlert UP:

Uptime (www.farces.com) is UP again at 09/09/2018 06:02:25 PM, after 15m of downtime.

During these outages, no internet traffic — inbound or outbound — is passed; local-area network traffic passes normally.

Please advise.