Mail app shows endless "connecting" till reboot

Since iOS 8, I have noticed a random occurrence with my iPhone 5S mail app. It happens about twice a week. At home, on wifi, it gets new mail just fine. However, once I'm away from wifi and try to download new email, I get the "Connecting..." message and it hangs there. If I turn my phone off completely and then back on, it connects and downloads everything. What's going on?