gray screen , no boot from usb

I know its an old computer - but for me it worked just great until now: after starting and showing the apple logo there is only a white screen without any cursor or signs.

It is possible to boot in safe mode - and everything works fine.

Hardware test passed, hdd rights fixed.

(no wifi module available)

ehternet works

but still no standard boot possible

trying to boot in diagnostic mode (CMD-D) - white screen

resetting NVRAM an Pram

resetting SCM

but still no standard boot possible

booting in verbose mode:

errors: "unable to open /var/db/bootCaches//PreheatedUser/Merged.playlist: 2 no such file or directory

trying to install new system (lion) with usb_stick and from Timemachinebackup and form Web: (alt)

Usb-Stick: shows boot options (HDD, HD Recovery, USB Stick (bootable with lion)

starts reading usb-stick then screen gets black for 1/10 sec. and then - the white screen again.

Switched Memory - no change

Opended Mac and plugged off several devices (one after the other) tried to boot.

WIFI- Modul

Bluetooth Module

SuperDrive

HardDrive

but still no standard boot possible

any ideas? thank you very much