Octave pedal with good tracking on low E?

[Inazone]

I've been experimenting with a DOD Octoplus for the past couple months, using it to run a clean guitar signal to a bass combo, down one octave to double bass lines on my guitar. The DOD actually has worked out fine for what I want to do, but it has trouble on the low E string. I tried out a Boss Octaver, and it was the same problem. I'm not using the octave down while double-picking the low E or doing anything fancy, but it still warbles.

Are there any other octave pedals that would do a better job, or is this a common problem? I've been told that some rackmount effects units work, but I need a pedal.