Please help me...

I have a Pioneer DVR-A05 (retail) and I used Nero 5.5.(8?) and about 50% of my DVD-Rs would get an error message at the end of the burning process which said:
"Illegal Medium - could not perform end of disc at once".
But this was no big problem because the discs played perfectly on my Sony DVP-S335 player. Even those with the error message.

But I tried to get rid of this problem anyway by upgrading to Nero 6.0 Demo. I uninstalled the previous version, installed Nero 6.0 and now Nero hangs with EVERY DVD-R I want to write. I even have to RESET my computer! It has no problems with any CD-R's, they burn perfectly! But it hangs with every Data DVD or Video DVD I try. When I start writing he says that he is "Checking the files, allocating the files, ..." and then he does nothing. He doesn't even write 1%!

What could be causing this? I uninstalled Nero 6.0 and installed Nero 5.5.10 and it gives the same problem as version 6.0. So I don't know what to try anymore...

Any suggestions?