Thanks for the responses so far (including the differences between IPR and software), though I don't see any tutorial. Plus, I'm working through Maya's in-built tutorials, so I've not adjusted anything that should be causing problems. No bump maps - the wall is simply a coloured rectangle, the ground only has a procedural texture applied. I've just tried rendering a sequence, and it doesn't happen to all the frames - just seems to occur randomly (around 10 frames out of 50). All the settings are default, though the image sequence I've just rendered was Maya software, and I tried a couple of resolutions. The results were the same. I've not adjusted any paramaters. Any ideas?