I came across this same problem, though on amd64, a couple weeks ago when installing to my "new" build, on an ECS/Nvidia AM2+ motherboard with BIOS. At first I thought it was USB related so I burnt a CD with 12.1-RELEASE. Same thing. Then I burned another one with 11.2-RELEASE. It worked, installation was smooth. Afterwards I upgraded straight to 12.1. No problems this far.