DVD stops playing after an hour and jumps back to beginning

I am using DVD Decrypter, DVD Shrink and Nero, but the majority of my DVD's after burning when played for an hour or so jump back to the beginning, am i doing something wrong??

 

As a guess, you may have a limited function application, allowing only one hour replay.

 

Thansk for the thought but why do some play the ful length of 2 hours or more and some reach approx 1 hour then jump back to either the start or 10 mins or so, only hapens on some discs.