I'm assuming you ordered the dvd set from one of the burner services that are available and if this is the case, even though they provide an excellent service the burner most likely used the CHEAPEST posible media that he/she could get there hands on and probably burned them at faster than recomended speed and now what you are experiencing are read errors, if this is the case the best thing to do is try cleaning the dvd and your dvdrom drive or try another drive
"some drives are very picky when trying to read DVD-R/+R discs"
If none of this works then the disc is no good, order another set but buy quality media and send it to the burner and request it not be burned any faster than 4X speed
Good luck
Rodney