You did everything absolutely the right way, burning a 8x media at 4x. It is safer.
Knowing the answer to the questions, my suggestion would be to try some other software for backups and see what will be the result.
Please, make a try with AnyDVD and CloneDVD2 or DVDShrink and Nero, burn at a lower speed (as you have done). Let's see what happens with skipping.