The warranty is your friend
My 2008 MP came with an Optiarc 7170 unit. It never worked well and had the same symptoms as yours. In addition it wouldn't burn CDs or DVDs consistently. I tried alternative firmware revisions without success. I finally called Apple, and it was clear that I was one of many who had reported the same issues. They immediatley shipped me a Pioneer DVR-112 which I installed myself and it's worked flawlessly. I'm amazed they're still putting these pieces of junk into the MP.
If Apple replaces your units with Pioneers, note that you have to remove the front bevel (branded "Pioneer") from the tray due to it being slightly too large to fit comfortably in the MP's optical drive bays. It's only lightly attached to the tray and comes off with very little force. It has no functional value, and if you don't remove it the optical drive bay doors will not close properly. If you don't feel comfortable doing this yourself, take your MP to the closest Apple Store or Apple Certified Tech and have them install the drives. Make sure they test opening and closing the trays before you sign off on the repair. It should all be covered by your warranty.