Go back one more.. 7.5.2 or to 7.6.1 as the best of each version firmware at the moment.
Do a full factory reset of the TC and start from scratch..
When you do the pppoe settings on the TC.. update .. then shut off the network again.. reboot the modem.. then wait and reboot the TC once the modem stablises. Sometimes pppoe is a one shot deal.. miss the first time around and it can take ages to reset. If it doesn't work try again.. rebooting both modem and TC.
Frankly I am surprised the update hasn't been pulled. It is just not up to scratch. One wonders how much testing Apple is doing these days.