My Mbps keeps freezing! Help!

Hey everyone! I have a mid 2009 MacBook pro unibody and it is currently running on leopard. My problem started a couple of months ago. I would use my computer and it would freeze. I would get the spinning beach all of death and then I would have no choice to press the power button to shut it off. It got to the point where it was not booting up. I decided to try booting up from my leopard install disk and running disk utility to see if there were any problems with my hd. It turns out that there were no problems with my hd, but I clicked repair anyways just in case. My computer worked fine for 15 minutes and then started doing the same thing. I then tried resetting my pram, that helped for all of about 20 minutes. I then decided to save all my data via FireWire to my sister's MacBook and decided to cut my losses and erase and install leopard again. That seemed to do the trick! I did this at the end of March and it has worked perfectly since then, until now. It is now freezing and not booting up again. I ran disk utility again and there is no problem with my hd so now I am extremely confused. I wonder if any of you have had this problem? If so, how can I fix it? I am DESPARATE! I cannot afford another computer and I need it functioning for university! ANY help at all would be greatly appreciated! Thanks! Sorry this is so long! I had to use my mom's ipad!