Did you also reboot the TC? What mode is the TC running, as it seems strange for your wifi to drop out.. the self-assigned IP means it simply hasn't got dhcp working.
I will have to search but there is a thread of people having issues with TC and these later Motorola cable modems.. they seem to not work particularly well.
If it has dual NAT then you should bridge one device or the other.. I do not know how the 6141 goes, but a lot of new cable modem routers have no bridge setting as such but you simply turn off NAT.
You can certainly bridge the TC easily.
Double NAT causes issues and you are best getting it back to a more ordinary setup with one NAT router.
After my initial post I decided to just reset both the modem and the TC. I shut both down. Powered up the modem, and once that showed it was working as per the lights ladder, I powered up the TC and did a reset to factory defaults. After a short while all was back to normal without any intervention on my part.
So my take away: with a Motorola SB6141 and a TC just reset both when you have a problem like I had.
This may not work with all TCs and all Motorola modems, but for my match up it does. It's been running fine now for a couple hours. I will leave it run overnight and see if it holds.
Thanks to LaPastenague for the rapid reply and helpful comments. I appreciate that very much.