DMA is not related with external drives.
If the external drive is connected via the USB port, check that there is no CPU intensive process running when you are burning, because the USB port uses the CPU to transfer data.
Check also that you are using quality discs. A burn that stops before the end most of times is due to a bad quality disc.
BTW, it is also possible that after two years the drive is actually going bad. To be sure, can you check the drive on a different computer and using proven quality media (verbatim for example)?