I have a 2014 Specialized Sirrus Carbon with Tiagra front and rear derailleurs. Several times the chain has come off the front derailleur—a somewhat worrisome notion when you are clipped in and going slowly up a hill. Thankfully I didn't fall. It has me thinking an upgrade is necessary. My other bike has Dura-Ace derailleurs and I've never had a problem with the chain coming off. I'm thinking of just getting a Dura-Ace front derailleur to solve the chain problem. Do I have to get both derailleurs done, or can I just do the front?