Yes, tried all different media. There *is* a small difference in media and error rates, and possibly speeds, but nothing obvious.
To me, it seems like a process which is running and which is interfering with the data, either by corrupting it or by locking up the CPU for a while every so often.
I do see a strong improvement if I reboot the PC and do nothing before writing the DVD.
The daft thing is that I have just written a dual-layer DVD with a 6GB file, on an old 800MHz dell laptop with a USB2-attached Plextor writer, connected via wifi to a router which has an ethernet-connected 250GB network hard drive, on which the said file resides. So, the data travels over ethernet, over wifi (and 11b/g is actually only 11b when running P2P data), over USB, and gets written to a 2.4x speed dual layer DVD. And it worked just fine, verified fine. Technology is great when it works.