I have the same issue with that burner and Verbatim -R discs.
It seems to happen only at 6x speed. The first time I saw the write speed drop to zero and stay there for some seconds I just waited for the nice little window that tells you about a write/I/O error to show up, but it never did, instead the burning continued and rise to 6x as it should. It happens around in the middle of the burn. A disc quality scan shows no issues at all. My guess is that there is some type of re calibration going on.