I had the exact same problem months ago and still nobody has been able to figure out what is wrong. I took it to a shop for a diagnostic but they refused to tell me what was wrong until I paid them over $130 to fix it. Apparently all they did was fix my old crappy PSU but when I got the computer home nothing had changed. I took it back, they kept it for a few weeks, only to finally tell me that they didn't really know what was wrong (they said they "thought" it was the motherboard), and suggested that I just leave my computer on all the time. Although I should have blown up their store at this point - or at least asked for my money back
, I just wanted my computer, so I took it home and once I got it started up I just left it on. That worked for a while, but now my computer just freezes up anytime I try and do anything more complicated than surf the net. It seems very difficult to pinpoint the problem and fixing it would end up costing more than this computer is worth (if it got fixed at all) and I am reluctant to just start buying replacement parts only to find out that it wasn't what was causing the problem.
I hope you find a solution to this problem, because nobody I've talked to yet has a clue. Some say power supply, some say hard drive, some say motherboard, some say processor, some say heat sink, and some say RAM. So at least I'm pretty sure it isn't my CD-ROM or internal speaker........If you have the resources, time, and skill that I do not, you may be able to figure it out through simple process of elimination. Good luck.