OK, well so far it looks better, the highlighting problem seems not to happen within a minute as it did before. That's a lot of progress for me!
However, the attached screenshot does show up a local flight more than 2000nm away, it was over Burkina Faso when I looked. Also it seemed to be updating regularly, and since it was showing as an ATP I'm not sure if it was meant to have ADS-B or if this was some strange garble.
So, let's chalk this one up to something odd for now. And indeed it was. I've seen two ATPs this evening that show a position which is either 12, 24 or 36 degrees North and is always 0 degrees East. Looks like something wrong in the avionics on board, or a faulty setup.
I'll try and test for a decent period over the weekend.