1st rule, remove and reseat all the ROMs. almost 1/2 the time this is the issue. the ROM get's a ditty connection on one of the legs and stuff goes missing.
failing that, you may have a corrupted ROM. check to make sure all the ROM chips have their "windows" (if so equipped) covered with a sticker or tape or something to block the light out. any amount of light entering the ROM window can result in corrupted data.