Lewis, have you tried fitsverify on the file?
http://heasarc.gsfc.nasa.gov/docs/so...ls/fitsverify/
You'll need the cfitsio dll for which there is a link on that page, but FYI the link you want is
http://heasarc.gsfc.nasa.gov/fitsio/
It is possible that SX is writing a bad or incorrectly ordered header, or that PI doesn't like some perfectly valid entry.
EDIT: Make sure fitsverify.exe was built with the same version of code as cfitsio.dll. I had to rebuild it because the dll was newer than the one the prebuilt version used.