Yeah, it's definitely a unique problem to overcome. I've found that quite a few pianists fall into one of two categories: fast fingers or fast octaves, but almost never both. Not sure if it's because of the way they develop technique or what, but it certainly seems to be an interesting study.
I learned a lot of ragtime, particularly Scott Joplin, early on in my piano playing career, and I developed a real knack for the stride bass. So, octaves and leaps come very naturally and easily for me. But as you know, I didn't spend any time on Bach, and hardly any on Handel, Mozart or any other classical composer until very recently. So, the fingers suffered for many years.
When we talked about Rachmaninoff, I had a feeling at least one of them was the case, and was leaning towards #3. It seems to be very common, especially for pianists who aren't sure of octaves. I see it all the time when I watch people play. One way to start combatting it would be to play octave scales (or anything with octaves), and inbetween every octave, completely relax your hand. Will be slow practice for a while, but your hand will learn what it's supposed to do pretty quickly.