Thanks @Daniele_Sunrise - you and Sunrise have done a fantastic job.
Less than 48 hours after I posted this thread, a new modem had arrived. It only took a few minutes to get the new box connected and, so far, it’s been flawless. No internet speed drops, and no unexpected reboots at all.
Although there are one or two things I’ll miss, the new Connect Box 3 is a much better box than the Gigabox (Connect Box 2) it replaced. Thanks again.