Rapid mail open/close!

5s (3 months old) Ver. 7.0.4 (11B554a). Problem is this; in past few days this has started happening. Emails arrive, touch icon on bottom bar - mail flashes open, then immediately closes again. It is random - one touch is good, next time - 5 touches required? I did a restore from last backup (only 48hrs old) but problem still exists? Do I need to WIPE the phone and start from scratch with an reinstalled OS?? Any ideas? Thanks!