ok, my suggestion because I faced same problem wih my ASUS burner gives same solution, of replacing the burner. But for one thing Blank DVDs show zero space only. You can check it in nero express there is an option called Disc Info which gives size of the disc and free area info.
2nd comes DMA reset for burner if it taking very long to burn dics
burning at 16x time taken to burn is 6.5-7 minutes
12x in 8.5-10 minutes
8x 13-16 minutes
These are normal timings compare with your timings
3rd is a precaution you need to take while burning DVDs, while burning a DVD it is recommended not to do anything else, it can cause problems which you are getting.
Multi-tasking should be avoided while burning discs. People will say no it does not effects, that is why I am telling this beforehand that this does.