I've got a problem with dvd+rw-tools (version 5.21.4.10.8-1) on amd64, gcc3-4: If I try to write an 8x DVD-R (Verbatim), then the writer (LG GSA-4082B, firmware A208) starts with 4x as expected, but later it drops to some arbitrary value (about 2x). After a while it goes up to 4x again, but it never reaches the promissed 8x.
Keep in mind that if system for whatever reason can not maitain data flow at required rate, unit will have to take idle revolutions and depending on firmware implementation performance loss can be as abrupt as you describe.
Below you can find a sample session. Writing the old Verbatim DVD-R 4x medias was never a problem. If I use "growisofs -speed=4" then there is no problem with the 8x medias, either (except for the lost performance).
Given the speed descriptors presented below you should try -speed=1 to tell you unit to limit recording velocity at 4x for 8x media...
GET [CURRENT] PERFORMANCE: ~ Speed Descriptor#0: ... W@8.0x1385=11080KB/s ~ Speed Descriptor#1: ... W@4.0x1385=5540KB/s
It's known problem which growisofs (it interprets lowest speed reported by unit firmware as 1x and your unit offers only 4x and 8x for this media), which will be fixed in upcoming release. A.