Also to see if its a Carrara and the Tiff format version that GIMP uses, try this.  Get irfanview (one of the best little image viewers/resizers etc and its free, at http://www.irfanview.com/

Open the file and save as/export to .tif again, also save out to another lossless format say PNG or BMP, then open that file and re-save as TIFF and try each one, to see if it has the same issue.  if it doesn't then it narrows down what is happening.  I would still like to try it, if you can put up a sample somewhere.