Huw
Some interesting responses - it does seem to be more mac related - although the size of your png's may be a factor (?)
If its a hire machine - Id definitely ask for a replacement - especially if your backup seems to be working fine. Without wanting to steal the job - I do have hire catalysts available if you get totally stuck - But do go back to your hire company first!
Try emailing the files directly to me and I'll take a look.
One point however - since you are only outputting 3 x 1024x768 through the TH2G-D, your still images don't need to be any larger than 3072x768 or 1024x768 each.
If you are blending the whole 3072x768 - use x-offset in each mix - or try splitting the 3072x768 file in to 3 x 1024x768 and use 3 layers and 3 mixes to re-construct the image. (see example pics attached)
Ignore the bottom %bar for stills!...
Simon