Intel Mac :: DVD Player Doesn't Recognise DVDs. Keeps Ejecting?

I've been having problems with my iMac recently, more precisely with my DVD drive. I think it's a software problem, and no mechanical problem. Let me explain: 

When I insert a DVD:

- it will sometimes work perfectly. I can read, burn or whatever I feel like…

- it will sometimes not work. I don't know why, but the drive gives a few try (mechanical noises), and than ejects the DVD. If I try a few more times to reinsert it, it will sometimes just keep the DVD and not eject it, without recognizing it. When I try to open the Disk Utility program, it will simply froze. Or it will open after 5 minutes, but the DVD player will not exist and not appear. 

I need to restart the computer. Sometimes it will work after the restart. Sometimes not. It's really random, and I haven't found out on what it depends… Sometimes, when I reboot it because of a trapped-unrecognized DVD, the DVD player will not open. "Initialization error [-70012]"… 

I've tried Option-Cmd-P-R at the Start, but it didn't help… I feel like there's something wrong with the software. 

It's a 2 years old iMac, using Snow Leopard (Mac OS X 10.6.8). 

I've tried looking on the net, but a lot of people are experiencing mechanical problems (it simply never works), which I think is not the case here (it does randomly work perfectly).

Info:
iMac (24-inch Early 2009), Mac OS X (10.6.8)