This is a long shot and i havent investigated it deeply, but this is what works and doesn't work:
The 40MP files are Nikon D800's, the 80MP is an PhaseOne IIQ.
80 megapixel image + OpenMP = OK
40 megapixel image +OpenMP = OK
80 megapixel image +Openmp = NOT OK, image has vertical stripes, takes ages to process and looks utterly corrupt
80 megapixel image (no openmp) = OK
Does this ring a bell anywhere?