What is wrong with my Playstation 3?

So I first bought a refurbished 40g PS3 from gamestop. Beat god of war 1 and 2 on it then started to play GOW 3 and it froze 4 hrs in and lost everything and gave me the error code that it didn't read blue ray any longer. So I returned it because it was less then 30 days and they gave me another PS3. Incidentally they gave me another 40g but when I opened the box it was a 80g. Crazy mix up but I wasn't complaining. So I was planning GOW 3 and it froze up again and wouldn't read the disc any longer. I then took it back and they gave me another 80g because that's what I took back to the store. I haven't had time to play on it really yet. I am just trying to figure what is happening to my PS3s. It is sitting on a open glass shelf with nothing else and has plenty of ventilation. I was thinking maybe its the new 3.65 firmware. The guy at gamestop said maybe its the disc but I am not sure. I am going to update it to the 3.66 FW tonight and finish the game off. Just wanted to see what everybody thought because I do not have that much experience with PS3 because I sold my xbox 360 elite about a month ago to purchase an ps3 to try something different and new.