You seem to be doing everything right. A few things come to mind that you could check, just as a last resort.
- is the G-rex daughter-board fully plugged in into the motherboard connector?
- is the G-rex daughter-board's small connector fully plugged in into CSMKIII/CSPPC's video card connector?
Using taller spacers (such as the original ones for Escom A4000T units) or custom made ones that are a few mm taller than the regular sized ones will push the CSPPC/MKIII a little better into the Grex and make a better fit. Perhaps that is it...just guessing.
Also, does your Grex daughter-board have a wire that was hand soldered in order to fix a problem with a missing trace on the PCB? IIRC, mine does, which DCE used to fix a missing trace that was never made originally when the PCBs were printed. It will look similar to their solution for Grex 1200 new version, which makes your Grex work with DMA in two slots. :-)