If I understand it correctly, the drive's firmware looks at the disc and tells Nero what speed to burn with. Assuming you have not limited it yourself, can you do a drive check somewhere to check the media or media check, whatever might work?
Is your media rated for faster than 2.4x? Have you checked for any firmware updates for your burner? Have you tried a different type of media?
If you can find a NeroHistory log, you might post it, after you have removed your serial number.