[ubuntu] Monitors occasionaly go blank and then half-flash

I have an intermittent problem which happens anything from one every few hours to once every few minutes. It looks like this:

One of my coected monitors (always one that's coected over Display Port, never the one coected over VGA) goes blank, and then every few second the bottom half of it flashes with what might be, but I'm not sure, a mangled version of what the whole monitor should be displaying.

This problem sometimes seems as if it's more common if I'm actually using that monitor actively, particularly if I'm typing something on it, thought I don't know that for a fact.

I've swapped cables around a few times to rule out hardware issues, but so far all I've confirmed is it's always a Display Port coected monitor that fails.

I thought perhaps some log file might be helpful in working out what's going on - can anybody point me to the right one?

For the curious, I'm using a Lenovo Thinkpad T450 laptop in a dock. I did have three screens set up - two display port and one VGA, though after a restart and cable swap trying to fix/lea more about the above problem now only the VGA and one of the DP screens are detected.