3500AG nero buffer problems

Hi!
I’m using 3500AG, L&D 2.TG;
Recently I have some problems burning dvd-s with it (Verbatims DVD-R)

Unique Disc Identifier : [DVD-R:MCC 03RG20 ]
Disc & Book Type : [DVD-R] - [DVD-R]
Manufacturer Name : [Mitsubishi Chemical Corp.]
Manufacturer ID : [MCC 03RG20 ]

First it gave me dvd seek… error, I thought it was because I have got some mess on my drive, than it stopped, I was able to burn with 12x speed;
Later it started to behave strange, when burning with nero buffer is going crazy sometimes (droping to 7% and stays for a little while and than back to 100%) - I know it might because of the writing method, when going for higher speed it may cause a slight glitch in buffer, but for me it’s too long than the drive starts to spin fast and is burning again, and this goes for several times but when I scan the disc after burning with nero dvd-cd speed for quality it gives me some bad sectors which the drive can’t read so I can’t copy the data:(

The disc looks like this:

The circles can be clearly seen
I have to add that this ain’t happening all the time (yesterday I burnt 3 dvd’s and ended with one bad, today I burnt 2 more and also one bad)
Can anyone help me, please
I forgot to add that now I;m burning with 6x speed and it still happens:(