Hi Niel,
By any chance, do you have access to the render logs for each frame? For example, if you happen to be using Deadline as a render controller, you can often look at the render logs to see where a problem might be occurring. In this case, we're curious if the same version of Terrgen is being used by the render nodes rendering the incorrect frames, as the version being used by render nodes rendering frames correctly. Also, the render logs may show errors when trying to load the files.
Would it be possible for you to send us a couple of consecutive frames, like 72 and 73, in which one frame renders fine and the next does not, so we can compare the two? This may help us determine where the problem and what's changing between frames.
Does the render always fail on the same frames, or is it different each time the shot is rendered?
Are you using the Colour Management feature, and if so, is the render nodes which are failing to render the image correctly, finding the colour management config files? If the OCIO environment variable is being used at your studio, has it been set on the render node?