Update ImgBurn. Try burning at 4x. Try cleaning the drive with a lens cleaning disc, or even manually if you know how to do it.
If all that doesn't help then you could try finding the 2.4x rated Verbatim DVD+R DL blanks, or perhaps get an external, non-slimline burner, e.g., an Optiarc AD-7240S or 7260S in an external enclosure.