Hrm, interesting. Have you done the standard checks of the Power Supply and memory? I see you have swithced out HDD, so its probably not the instance of the OS install. My best bet would be on the memory... When you load my computer or click the start button, the OS has to move things into mem to display stuff and what not, and if your memory is borked, that would explain the hangups
Try some other sticks of ram and see what happens
Good luck, keep us updated