T/D Flashing over PFD before descending

Good day, after the purchase of the 777 (I don't know if it's connected or not), i've noted in my NGX one little "problem"

In fact before descending the T/D green write over my route in the PFD start flashing fastly or moving up and down fastly.

the airplane is able to descend and the path is always correct, but I've never noted this kind of flashing or moving of T/D, it was every time fixed and stable in my route.

Any Suggestions?

Cheers