My Son's Dodgy Playstation 2

My son's had a Playstation 2 console for approximately 3 years and to be honest he's had his moneys worth out of it, but over the last couple of months it's started to be a bit hit and miss on the disc's it will play.
It started off by just sticking on a few games but has now moved onto not even loading most of the games, yet it will play DVD movies ok. On loading up a game everything seems to be running fine then it just sticks and will go no further.
On the games that it's decided it's not going to entertain, it has left it's mark by leaving scuff marks as though the cd might be catching on something?.
With the console being well past the Guarentee expiry, I decided enough was enough and opened it up to give it a good cleansing! as I did this I had a good look around the innards of the beast and had a good poke around trying to find anything that might leave the scuff marks.
Alas, I couldn't find anything and the tempermental bitch continues to ruin my son's enjoyment. If anybody has any suggestions on how to fix these problems please contact me.