Commonly for some reason certain servers such as the ones being sued by Romulation will corrupt a fraction of data in a file... Causing ISOs to screw up.
I've been with these fellas since nearly the beginning, and have downloaded thousands of files from them.
And some of them, spiecally those running over 200+ mb, I had to Re-Download.
So if you have the correct specs, and the emulator is for sure properlly configged.