It might very well be the Memorex drive dying. However, the Memorex firmware on that unit is well-known to be finicky and problematic. The good news is those are re-badged drives, meaning Memorex bought LiteOn-manufactured drives and slapped their pretty sticker on it to sell it.
All that needs to be done is for the firmware to be cross-flashed to make it a true LiteOn drive. The correct firmware to do this is located at http://codeguys.rpc1.org/firmwares/dvdrw/1693S.KS0B.patched-cf.rar
The RAR file should include all of the documentation required to patch the firmware.
Like I said, the answer was going to be a bit complicated. But the drive may very well be perfectly fine, just in need of this somewhat-easy update.
If you want an easier, yet more expensive approach, then you can still buy a new drive, yet. I would highly suggest buying a Pioneer drive and nothing else. Just Pioneer. Any Pioneer.