Ok. Thanks for that. I thought that might be the case. The manufacturer specs must be incorrect. I guess I'll just have to buy the more pricey Verbatim media, as these ones don't like even a 4x burn .... 50% coasters, but fine at 2.4x.
For the record (and other members) these are Verbatim 8x DVD+R DL "Life Series" with printable hub.