iPod in Car using FM and Aux

I've been using the Griffin Aux cord for my iPod Touch to direct into my car stereo and it's been great. However, I bought this exact FM transmitter from eBay because I wanted to charge and cradle the iPod while driving as well. The problem is songs keep cutting out about every 45 seconds when the entire set-up is plugged in. Can I not run an FM transmitter and Aux cable at the same time even though my stereo is tuned to Aux and not FM? Is the charger activating the FM feature and interrupting?