Welcome to the forum
From reading other people’s experiences and opinions of Matshita drives, they’re known to be incredibly media-picky, and most (if not all) of them just aren’t good drives anyway.
I would suggest either of two things:
Buy better DL discs (Verbatim +R DL Made in Singapore are the best you can get, but still no guarantees how your Matshita would handle them).
Buy yourself a nice new external drive. I would recommend Samsung if you’re planning to keep those Memorex discs. If using Verbatim DL, practically any newer external drive should do the job.
Hopefully other members will have a few more suggestions for you.