Well, optocouplers generally aren't very linear - they're either ON, or struggling to turn off. This isn't exactly the case, but once you've fiddled around with optocouplers/optoisolators for awhile you will find that the turn-off time can seem to be an eternity.
If it's an option, convert the analog data to digital, and then send it through an optocoupler/optoisolator. You'll eliminate many problems by doing that.