Unfortunately, you may have a defective dvd unit.
Another thought would dictate trying a different brand of DVDRW
If trying a different brand makes no difference, I would open a report with Acer for a defective drive and get them to replace it, if it's still under warranty.
Another option, if the drive is defective and not still under warranty, is to buy another type on ebay (or similar), making sure it is the same interface, i.e. pata/sata, and just transfer the faceplate from the defective unit to the new one.