Modem randomly reboots

Here is our setup:
We have static ip addresses and the modem is in the bridged mode.
We have only 3 devices on the network:
1. Computer A with a web server and a dns server running Windows 10 pro.
2. Computer B with a web server, dns secondary server, and an email server running Windows 10 pro.
3. A netgear wireless router providing wireless to customers

Last week we moved from DSL to Comcast cable. Since the cable was installed we have an issue where the modem will reboot randomly.
Sometimes two or three times an hour, other times it may go several hours before rebooting.
We never had this issue with the DSL and the computers we had with DSL have not changed.

Comcast has changed the modem (Cisco DPC3941B) and tested the line thoroughly. Our upstream and downstream levels are perfect and they have not fluxuated for the entire time.
Our building has several other comcast customers and they have monitored them and us at the same time and none of them go down when ours reboots.

After ruling out the cable and modem as an issue, the next thing we did is look at our machines.

We removed the netgear wireless router but the reboot still occured.
Next, leaving the wireless router off, we removed Computer A, and soon the reboot again occured.
Next, leaving both the wireless router and Comptuer A off the network, we removed Computer B and waited. After 2 hours there was no reboot.

Based on this we conclued that Computer B is having an issue with the modem.
On Machine B is we shut down the Web server, DNS server, and Mail server the reboots seem to stop (we give 5 hours for a test as this is the longest we have gone without reboot since the beginning)

We then try turning on the mail server, and soon a reboot.
We disable the mail server and turn on the DNS server and soon a reboot.
We disable the DNS server and turn on the email server and soon a reboot.

No matter which server runs (alone) the reboots begin again.

We have scanned the machine for virus, malware and rootkit (as we do very often) and it is clean.

We have disconnected the ethernet and connected via wireless to the modem with the same result, so a faulty nic card has been ruled out.

Comcast has sent techs out 6 times in the 8 days we have had their service and they cannot find the problem.
One tech suggested "maybe it is an attack", so we set the network monitor on and waited for an attack and the network usage was unchanged immediately before a reboot.
Also it seems unlikely that an attack would reboot the modem, the modem logs only show the modem getting its address after it powers up again.
No other info is in the modem logs.

Can anyone suggest ideas on how to isolate this problem? Again the same equipment functioned over DSL without any issues for years.

Although comcast seems to be doing all they can (tech sat in my office for several hours Saturday morning as we monitored and tried different things unsecussfully), we obviously cannot functions with random 5 minute drops throughout the day. We process our credit cards through Computer B and when it goes down we are out of business until it comes back up.