Try burning at 4x the whole way and see if you get the same errors levels that you are on that last 300mb.

Maybe the drive is just awful at burning those discs at 4x.

As to why the speed is dropping... well that's anyone's guess.

You could try switching the 'Write Type' to incremental, that sometimes makes a difference - depending on the drive. All being well, DAO and Incremental should perform equally.

I can assure you it's nothing to do with ImgBurn though - so to speak. Your drive controls the burn quality - end of story.