1027x768 lockups on G400 DH

[Marmita]

I managed to tune the 5.21 drivers as ant described... its working fine. more or less at the 5.13 speed BUT it randomly locks up in high res modes like 1024x768. (Q3Test 1.08). tried to turn off the sound card and remove bus mastaring but it didn't help. andI didn't use any kind of o/c. does anyone have any idea what's going on ?

PIII - 500Mhz
ASUS P2B Bios ver 1010
G400 DH
Adaptec 2940U2W SCSI with a seagate Cheetah 9.1GB
SoundMonster MX300
ISA ISDN CARD (donno the firm)
3Com EtherNet III ISA network card
Plextor UltraPlex x32 SCSI cd-rom