I've seen this on a number of occasions. Other aircraft 2000 ft below and catching up with us, the TCAS target starts to move around when the other aircraft is approaching directly below us. It often moves rapidly inside a radius of about one mile, disappears completely, pops up again, moves around and finally stabilizes when the other aircraft is a mile or so ahead of us.
First time I actually thought we had a playful fighter on our hands, but we soon realized it must be a spot where TCAS has some problems detecting the other aircraft.