DVD Speed Problem

I have an internal LG DVD Burner (in addition to an external). The internal burner is acting wierd and i'm not sure why. I burn DL DVD's and the speed seems to be inaccurate. First, I used a "2.4 X" disc and it indicated that it was burning at "4X"...which is impossible and inaccurate due to the fact that the 6 Gig movie to about 90 minutes. I use Nero and "2.4X" wasnt even listed as an option. Then I bought the "8x" discs and tried burning with them. It said it was burning at "8x" but it again to 90 minutes. My external is only capable of burning at 2.4 so i'd like to find out whats wrong with my internal? Any Help is greatly appreciated!