The newer BIOS supports a new RAM type. Some cards that came with the new BIOS will not work with the old BIOS; vice-versa also seems likely, although I have not observed this myself.
Is the RAM still Hynix?
015.
042.000.000.000000 supports:
4096 MB, GDDR5, Hynix H5GQ2H24AFR
4096 MB, GDDR5, Elpida EDW2032BBBG_DEBUG2
015.
043.000.001.000000 supports:
4096 MB, GDDR5, Hynix H5GQ2H24AFR
4096 MB, GDDR5, Hynix H5GC2H24BFR
The rest of the BIOS appears to be majority the same thing.