In case this is an issue, I'll mention it. I did this when I first switched to Rapid Fire shifters. You can't really shift both front and rear at once. Shift one, then the other. Once I figured this out, no more chain drop. Maybe this is waaaayy too stupid to even mention, but hey, I'm ok with admitting my own learning curve, which was STTTTEEEEEEPPP when I started biking. I have no idea how experienced you are, so no offense intended......