X and Fluxbox :: A werid problem with X

Hey, folks

I have been confused by the X problem for quite a while. When I boot DSL from the CD with option fb800x600, it works all right. But no luck with USB boot.
During booting from the USB flask drive, it looks normal. But after trying to startx, the green is left grey ( not totally black). ( The USB drive was made with the same boot option fb800x600) If I switched with ctl-atl-f1, I could see two tiny indentical terminals with each only about a quater of the screen. And I could even start X windows in that small terminal. Apparently, the GUI is also of that size and not readable.

Am I missing something?
Why the USB boot couldn't pause for option input like CD boot does?

My configuration:
DSL version: 1.2.1
Dell precision workstation 360, Bios A08
PNY 128M flash drive

Thanks a lot!

Shane

original here.