Mac Flashing Folder/Question Mark Startup

I've had this problem for possibly 4 months now and it is super annoying! It all started in the middle of Febuary. I started up my mac and up came the flashing folder. So I unplugged my mac and everything worked fine. Maybe a month later the real issue began. I started up the mac and the same thing happened. I unplugged my imac then plugged it back in and started it up. Same thing. I waited maybe 15 minutes and tried again. Then 30 minutes. Eventually it worked. Almost every time I start up my imac it gives me the folder/question mark, and I sigh to myself, because I know I have to wait like 30 minutes for it to work again. I called apple and they said it was something to do with a corrupt file, but there are none. I even had someone come over and check. This problem is really frusturating!

Here is my info:
Model Name: iMac
Model Identifier: iMac8,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.66 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 6 MB
Memory: 2 GB
Bus Speed: 1.07 GHz
Boot ROM Version: IM81.00C1.B00
SMC Version (system): 1.29f1
Serial Number (system): -cut-
Hardware UUID: -cut-


System Version: Mac OS X 10.5.8 (9L31a)
Kernel Version: Darwin 9.8.0
Boot Volume: Macintosh HD
Boot Mode: Normal
Computer Name: -cut-
User Name: -cut-
Time since boot: 10 minutes


It's a 10.5.8 Leopard too.

I really want help!

I also do not have the installation disk, so don't suggest anything about that.

Also my mac sometimes freezes when it wakes from sleep or screensaver. That's when i get pissed cause I know I have to wait 30 minutes again and I LOSE ALL MY FRICKEN WORK!!! GAAHH!

I have read all that support stuff and nothing worked. Also i can't order one.

I don't wish to reveal anymore information than needed