Allow me to admit to this, I had a very large case and I used some 24" long, flat, IDE cables without a issue.
Also, I am curently using 24" long, round cables without a problem.
However, when I made the switch, I knew that my optical drives were operating fine so I would know it immediately if the cable switch introduced the problem.
For a test, I would try moving the optical drive out of it's natural location to where it can be reached with an 18" flat cable.
Stick it outside, but along side the case.
Prop it up on a box or book.
In so doing you can eliminate the cable as the problem source.