Dw1650 low write speed and jumping scan speed

Hi. I’m using DW 8x16x8x16 (reflashed to DW1650). When writing Verbatim DL LS (MKM-003-00) at 4x, the real speed goes crazy:

It’s similar on 6x too, first half is usually at 2.4x. I tried to turn WOPC off, speed was stable, but I got a coaster.

Then I try to scan it for errors, it’s more or less ok, but what’s that speed fluctuation?

It seems that it’s not time based, but data amount based (periods increase when speed increases). Other DVD discs scan also looks similar.