Thanks; I got your e-mail. Based on what you have shown me, I think that it is a media problem. Most recent drives decide what speed they think is "safe" for the media; that is, the drive tries to select the speed that will optimize the recording quality. I think your drive is slowing down because it thinks that the media can't be burned at 40x.
One other thing you could try is updating the firmware to the latest version. This could help because new firmware releases sometimes update the way the drive selects the optimal recording speed.
If the process of updating the firmware fails, though, it can cause problems, so do it at your own risk. You might find firmware updates on the Dell web site or at http://www.firmware-flash.com