I recently went through this myself.... phlashing up to 1.05... going back down to 1.04.. and then back up again. Based on my fuzzy memory, the quality of your boot diskette is important, and not having HIMEM installed / running is a must. The ROM image that I got from the Fujitsu web site was fine though.
I should note that upgrading to 1.05 isn't really going to buy you anything... unless you're running a Symbol hand scanner. IIRC, that's the only change that they made from 1.04 -> 1.05.