ios 8 and imessage failure

Twice in the past few days I've had to restore my daughter's Iphone 5 due to imessage failing to work and won't turn back on. She recently updated to IOS 8 and we'd like to go back to ios 7 if possible.

Currently, If her phone drops below 20% battery, it automatically shuts down. When she goes to turn it back on, imessage will not turn back on (refuses to start) and more importantly, people texting her don't get a message the text didn't go through while on the receiving end, she's not getting any of the text messages.

This would SEEM to be a pretty major bug in IOS 8 and after restoring the phone, imessage works until the phone may run to auto shutdown again (below 20% battery). Anyone familiar with this or know of a 'fix'.

TIA