We received a new R640 with the X710, immediately after install we discovered the same problem with TX performance issues. As we are not observing this in production and only in the lab, we digged a bit deeper in the Non-contiguous TC - Disabling DCB message.
We disabled DCBX on our Juniper EX4550 (older software, 12.3R5.7), rebooted and all was clear. No more of these messages and performance are back to normal.