Maybe something happens during the burn that cause slowing. For example, the HDD containing data you are burning is accessed also by another software or by operative system (an antivirus scan or a software like e-mule or torrent are good examples).
Another possibility is that hard disk is so fragmented that the drive is not able to send data fast enough to the burner.
Another possibility is that you got a bad batch of media, so when the drive checks it during the burn it retrieves so many errors that the firmware decides to slow down the burning speed.
Can you run a test with cd-dvd speed?
First of all make an ISO file large enough to fill the entire DVD, then run cd-speed, select the "create disk" tab, select the "Burn Image" option, select a burning speed, and run the test. Then post here the image