|Installation problems can sometimes be a consequence of a dodgy (potentially failing) RAM stick; or to poor connections with the RAM edge connectors and the socket. Presuming you have two or more sticks involved; try Derek's method for cleaning RAM connectors...|
Remove both. Clean their edge contacts with a soft (pencil style ) eraser. Replace "one" stick firmly into its socket. Then remove it and re-install again into its socket. Do this two or three times so as to clean the internal connections on the socket.
Then try to install the OS.
If no joy (the OS won't install) repeat the above process with the another stick of RAM...