Murray...it is highly unlikely at this point that defragging or RAM memory has anything to do with it...in other words, the problem will be with some setting...

try this: Start a video playing, then right-click on the video and see if it has a little window for 'settings'...one of which asks "how much information can YouTube store on your computer" It may be that the default is smaller than most videos...If that doesn't help, try AOL 'help desk'...(It just could be that AOL, which is famous for controlling stuff, has some limiter in place.)