NEC 3520AW big problem with Nero 6.6

I have a problem when making second image of a DVD (it is not a dvd movie it is some my stuff) using NERO 6.6. Destination of the image is my HD it has around 100 GB free space.

When I make first image writing speed is good it writes between 9000-16000 KB/s and used read buffer in Nero is constantly at 0% then I burn this image on another dvd and it burns full speed. Now when making & burning of the first image is over I want to make another.

The problem now is that writing speed (when making image) is between 2900-5900 KB/s and used read buffer is 0% constantly. (it take’s much longer time to make an image). Again it burns full speed. When I shut down my computer I can again make one image with full speed but second has the same problem so I need to shut down my computer after I made one image if I want it to work at full speed.

My drive is secondary master and it is UDMA2. What I think is that my drive’s buffer remains full and that is why it’s speed reduces but when i shut it down it becomes empty and it works fine but only once.

Is there any way to solve this problem?