Are you running all your tests with the same pressed DL DVD? I’ve run into a couple of DVDs that show smooth but very slow transfer rate curves. I never found a reason but it occurs very rarely.
Check the IDE controller mode for the drive channel in device manager. With XP, right click on “My Computer”, select “Properties”, then “Hardware”, then “Device Manager”. In the pop up window, expand “IDE ATA/ATAPI controllers” with the “+” sign, then right click on the IDE (“Parallel ATA”) controler and select “Properties”. The click on the tabs for the Primary and Secondary channels to see the channel mode and info on what’s connected to it.
As I mentioned previously, as long as you’re seeing ~41 MB/sec burst transfer rate reported, this is not your issue but it’s probably worth trying the burst rate on your NEC and/or checking with this to see if anything has changed. (Your NEC will probably have a slower burst rate as it only supports DMA33 from what I remember (~20-25 MB/s)
MCSE has an option to increase rip speeds on the 115D. You’ll need to modify both the firmware and the Flasher to enable the same firmware version to be flashed. I found the higher rip speed caused the drive to slow down on many pressed DVDs right before the layer break and not speed up again during the second layer but it’s worth a try as a test and easily reversible.