Had a very similar problem on a sony vaio, random lockups many problem, tried to reinstall os but would never complete. tested mem it passed, tried different HD same, tried diferent cdrom same, tried diferent cpu chip from another system same. Tried memory from another system just for grins promblem cured. Put "bad" memory into other system but it seemed to work fine.
But at least both system working, thought oh well, wierd problem.
About a week later the other system began to have same problem, replaced memory both have been fine since. Can't garantee it but it sure seems like a memory problwm.