i have a probleme whit my sryrim..

my default setting is ultra but when i start the game i can play 20-30 min some time 1hours no lag everything perfect and then BOOM crash to desktop no warning no error message i try EVERYTHING close all application start game booster try in admin,windows xp pack 3 change my sound to 24bit 44.1 and all they other one^^ well etc etc etc .....alot of try and nothing...... when i putt it on low setting dont look to have any crash but really ugly graphique



theres my comp :
he have only 5month everything is new, all my other game run fine no crash to desktop just skyrim everything up to date

CPU: pentium 4 i5 2500k 4,8go
GPU: asus gtx 560 ti 1go
ram:gskill 8go ddr3 1600mhz
windows 7 64
p8p64 pro motherboard
xfx pro 750 watt
v6gt cooler

if any one can help me to solve my probleme thanks