I have an EHX Octave Multiplexer.
Hard to say exactly what your problem is without more details, but it sounds like it's picking up on other notes, harmonics, or just noise. It's a mono-phonic pedal, so make sure you're only hitting one note at a time; if the previous note is still sustained then it might "yodel" between them (yodelling is exactly what I call it).
Anything you can do to reduce noise-signal ratio pre-pedal might help. If it's part of a chain, where is it? Does your instrument sustain a note by itself or is your sustain achieved with an amp/chain? Does your instrument or playing style create harmonics, and if so do you want them? If you're playing with others might your pick-up be catching your bandmates?
I often find mine jumps around when I hit the note depending on how "clean" the start of the note is (how aggressive my right hand is, and whether I've hammered down with the left hand). Sometimes it opens up some cool character and effects, but usually I just want clean tone out of it.
Let me know how you get on in the comments, as I'm likely to find any tips for the same pedal useful too!