|As in #1 above plus test memory with Memtest.|
If memory tests OK as well as both hard drives, try setting BIOS to boot from one drive only (set one a bootable and the other as not), or keep both bootable but change the boot order. Experiment a bit to see what works. This will only work if each OS was the sole one when it was installed, keeping its own boot files on its own drive rather than using the same MBR.
You have to be a little bit crazy to keep you from going insane.