IMac hard drive dies for the second time... I think?!

Question

Hello,
My iMac has been behaving strangely for days now, constantly freezing and hanging. I've performed two clean installs of OS X 10.5, and one archive and install (and I'm doing another of them now). I think I better tell you what's been happening / happened to it:
First off you need to know that about 2-3 months ago it was sent off to Apple and had it's hard drive replaced under warranty, before that happened it was hanging for a day or so too, so when this started happening I was understandably worried - I quickly backed up the few things that weren't included in my regular TM backup.
Basically, it had been acting sluggishly and freezing for a few days so I decided to do an archive and install - it did nothing to help. Even so, the hardware test on the install DVD said that everything was fine (I ran permissions repair and disk repair in disk utility, as well as resetting the NVRAM, PRAM, and anything else I could think of).
By this point I was pretty much resigned to the fact that it was a hardware issue, but then I decided to boot into Windows 7 to see if it was still okay; it was. This seemed funny to me, because if something was physically wrong with it, shouldn't it affect the Boot Camp partition as well as the OS X one?
As it wasn't I figured that the Macintosh HD partition must be corrupted or something like that, so I proceeded to do a clean install and erase everything. After doing this it worked great, faster than it'd been in months! Unfortunately, after I'd updated everything back to 10.5.6 and installed my programmes / moved my files over it started behaving strangely again. Thus I came to the conclusion that there was some kind of corrupted file in my user folder / system library.
So rather than hunt it down I simply did another clean install. Once again everything was fine. I was much more careful about what I moved over, sticking to what I deemed 'safe' programmes; things like Adium, iLife / iWork, and Coversutra. I even updated everything (not via software update, but by downloading the packages directly from Apple). This seemed to go okay, and everythings' been fine for a few days but now it just completely won't work. Nothing changed since about 10 hours ago when it was still working. The last thing I installed was Safari 4 beta and some InputManager 'haxies' to make it better (adblock, 1password, keywurl). All of these things were things I've used for months and months so I don't see how they can be the cause... but still, they were the most recent things I installed, and Safari 4 is beta software (although I've never heard of it killing a hard drive / OS X install).
So here we are now. The last time I managed to get it to start it took >5 minutes to boot and froze incessantly upon managing that amazing feet, resulting in yet another hard reset by me. I left it to 'cool off' for half an hour or so and then booted up the install DVD and ran disk repair and permissions repair in Disk Utility and then tried to boot. It didn't work, I got tired of waiting about 10 minutes after starting it up. So I boot back onto the Leopard DVD and find that Disk utility thinks I don't have a hard drive... ah.
But then just to be annoying it decides after another half hour 'rest' and reboot onto the DVD that it in fact does have a hard drive in it, and even lets me begin another install of OS X! This is where I am now, about half way through that install. I have no idea why it is doing all this stuff, but I'm just glad I backup. I seriously doubt there is anything I can do to fix it myself (if anyone has any ideas I would really appreciate it?), and am loath to send it into Apple over Easter as I have a feeling it will take an abnormally long time to come back to me thanks to the holiday season.
If I do though, and they decide it is the hard drive do I have any chance of getting a whole new iMac as I really don't trust this one? This hard drive is brand new, replaced by them just a couple of months ago. And if it's not and the find that it works fine (which I'm sure it will after a clean install with nothing on there), that doesn't mean anything as, as soon as I use it and put anything on there - Bam! There it does again, and the truth is is really shouldn't be doing that kind of thing anyway. So really is there any chance they'll just replace it, because, while worth much less, they repaired my iPod a minor problem, but as it simply came back a few months later they just gave me a new one (a Classic no less, when it had been a 5.5G Video)... do they do the same kind of thing with Macs?
As really in summery, something is seriously wrong with my iMac - most likely the hard drive - and I have no idea how to fix it, even after trying everything I can think of. If someone has some brilliant idea that'll fix it I'll be glad to give it a go as I really have nothing to loose as all my data is safely tucked away on my Time Capsule, and I'll probably have to take it into Apple after Easter Sunday anyway (do I have any chance of convincing them to just give me a new one, and if they do, do that kinda thing how does it work, eg. mine is the top of the line 2007 model - 2.8ghz, 256mb, 500GB, 24" - will they replace it with the middle one that is closest to the specs of my old one, or the new top of the line that is closest to mine it terms of how much I paid for it compared to the new one?).
Sorry for the incredibly long question, and if it you read it all, or even just skimmed it thank you very much, any ideas / suggestions are appreciated,
Sam.
** If it makes any difference, my 4GB of RAM (2x2GB) is from Crucial, but I've been running that RAM for ~6 months no problems and the hardware test says it's fine.

Answer 1

My clean install worked out alright, although I don't think it really fixed anything, rather gave me a couple more days of use 'till I have to send it in. Someone please help my get it running properly again...