Yup!!! I Hve A C5 Torch Red One As Well! I Don't Know, But It Is Really Pissin Me Off, It Is Happening More Often Now And I Thought It Was My Ipod But Maybe Not! I Have Even Reset Using The Paper Instructions.
There are somethings that make the transmitter loose it. If you are out of range for a long period of time and so on. If you hold both the lock and unlock for 10 or 15 seconds it will re-learn and your good to go, until the next time that is. Once you hear the horn sound its ready. Or is this what you have been doing?