Personally I never install from disc. Sounds like your superdrive has gotten a bit fussy in its old age. If you've already got a legit licensed copy of Snow Leopard I would try and install from USB:

Using a machine with a good drive then you could make an image on that.

Fail that maybe someone can tell us if it's legal to download an image being as you already have the license. Optical media belongs in the history books