I was having the exact same problem with my x2lite+ for the longest of time, but I've finally figured it out. The bios that i was trying to use was the 256k m7 bios. I flashed that bios and it appeared to work, but every time I rebooted, the flashbios screen would appear once again.
It seems as though the x2 chip wants a 1024k bios file, not a 256 or 512 as you and I were using. I used Xecuter2 BIOS Manager to make my bios file 1MB (there are some good tuts out there on how to use it...forgot where i found it though, sorry). I re-burned my bios disc, did a flash, and it worked perfectly.
Hope this helps.