You can find out how to enable DMA by reading this.
If that doesn't help and you are prepared to buy a new CD-RW, try backing up your existing firmware with mtkflash, and flash to the newer version available from Dell (even if it says it's for the Optiplex). After all, you can't hurt anything if you're already prepared to throw it away. My guess is that that firmware version will work, even if it doesn't help the problem. And if it doesn't work, you can always restore the backup you made with mtkflash.
If you do end up needing a new CD-RW drive, I'd recommend an LTR-52327S. I have two of these drives, and they're really nice!