Your drive can only burn at those speeds on that media, regardles of what you want.
As you said you already tried 2x, 4x, 10x, 14x that means actualy the drive used 8x and probably 16x as the program sets the closest speed to what you select when it not match.
So, try one at 24x and if it not produce a good burn you have to try other media or get back to what you know it works.