'Virtual' dead pixels on MBP 15" display

Every now and then, my MBP exhibits 'virtual' dead pixels. They look just like ordinary dead pixels in that random pixels on the screen just stay white. However, they are 'virtual' in the sense that I can screenshot them and when I do an accessibility zoom, the 'dead' pixels zoom too.

This happens sporadically, but - I think - it's the same 2-3 pixels each time. It only tends to happen when I've laboured the graphics card for a while (e.g. plugged in to two external displays with graphics-intensive applications open), but they persist for a while after that.

This seems like a graphics card issue to me - has anyone else experienced this?