* have you tried a laptop or something in the new BT router connection. you may just have a bad router.
* also try a new cable between the router and its homeplug.
* try swapping the homeplug at the router with one of your other ones, it could be the homeplug at fault.
* try the moving the homeplug & the router directly into a mains socket and not the power extension strip.
I also use the homeplug in the same strip plug. I do use a "cheap" strip plug so no filters or lightning strike protection etc... This works for me using the 650 devolo versions.