Verbatim DVD-R Medium Write Error

I don't understand what I am doing wrong. I bought some Verbatim DVD-R's and in the beginning I had no problem burning images on it. Then suddenly it stopped working giving me a 'medium write error'. I entered a different type of DVD, Philips DVD+R and it burned. After that I switched back to Verbatim and it also burned on that DVD. Then some other time I wanted to burn another image on a Verbatim DVD, but again it constantly gave me a medium write error. So again I burned it first on a Philip DVD+R, which worked, and then tried it on Verbatim, but somehow now this trick didn't work anymore. After trying a lo-hot, rebooting, trying, trying, different settings, test burn, verify I don't know what it suddenly worked. And now it again constantly gives me a medium write error. How can I fix this?

Here is the log:
2009-03-20 23:21:15 +0100: Name : HL-DT-ST DVDRW GSA-S10N
Type : Optical Device

Disk Identifier : disk1
Device Name : HL-DT-ST DVDRW GSA-S10N Media
Connection Bus : ATA
Connection Type : Internal
Connection ID : Device 0
Device Tree : /PCI0/PATA@1F,1/PRID@0/@0:0
Total Capacity : 0 Bytes
Disk Number : 1
Partition Number : 0
Burn Support : Apple Shipping
Entry Path : disk1
Writes CD : Yes
Writes DVD : Yes
Media is present : Yes
Media is busy : No
Media Type : DVD-R
Media is blank : Yes
Media is appendable : Yes
Media is overwritable : Yes
Media is erasable : No
Media is reserved : Yes
Capabilities : Click for more information

2009-03-20 23:21:15 +0100:
2009-03-20 23:33:38 +0100: Burning Image “my image.iso”
2009-03-20 23:33:39 +0100: Additional information can be found in the ~/Library/Logs/DiscRecording.log log file.
2009-03-20 23:33:39 +0100: Unable to burn “my image.iso”. (No such file or directory)
2009-03-20 23:33:39 +0100:
2009-03-20 23:33:55 +0100: Burning Image “my image.iso”
2009-03-20 23:34:11 +0100: Image name: “my image.iso”
2009-03-20 23:34:11 +0100: Burn disc in: “HL-DT-ST DVDRW GSA-S10N”
2009-03-20 23:34:11 +0100: Erase disc before burning: No
2009-03-20 23:34:11 +0100: Leave disc appendable: No
2009-03-20 23:34:11 +0100: Verify burned data after burning: Yes
2009-03-20 23:34:11 +0100: Eject disc after burning
2009-03-20 23:34:11 +0100:
2009-03-20 23:34:12 +0100: Preparing data for burn
2009-03-20 23:34:12 +0100: Opening session
2009-03-20 23:34:12 +0100: Opening track
2009-03-20 23:34:12 +0100: Writing track
2009-03-20 23:34:37 +0100: Finishing burn
2009-03-20 23:35:37 +0100: Closing session
2009-03-20 23:35:37 +0100: Finishing burn
2009-03-20 23:35:39 +0100: Burn failed
2009-03-20 23:35:39 +0100: The burn failed because of a medium write error.
2009-03-20 23:37:26 +0100: Unable to burn “my image.iso”. (The burn failed because of a medium write error.)