I'll add my two pence worth too. Burning DVDs at 4X puts a hell of a load on the DVD laser. It has to run at max output to burn the media adequately. This can cause premature wear out of the laser, particularly in the low cost or older writers. Those lasers, and their optics, don't last forever. Much less so when being run at their max output which they will reach when near the end of the DVD. (The laser output has to increase if the surface speed where it's writing is going up.) So try burning at 2X. This should allow the laser to thoroughly dose the DVD at a lower drive level.
Or invest in a newer DVD burner. The latest ones use better lasers and optics. I've already had to replace one I bought two years ago. It slowly got worse and worse about writing until it quit entirely. I'm using a Memorex 16/4x dual layer now and it seems to use a constant surface speed method, I can hear the drive slowing as the burn track nears the DVD rim, and no problems.
Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."
System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70