I was having a bug with that. For some reason, it would randomly kick into that mode and I couldn't get it out of that. Not sure if it's the best solution, but killing the Settings process made it stop doing that.
I didn't even know the phone had a sleep mode, but my old phone used to go into a power-saving mode when it was searching for a signal for too long. It's supposed to stop the battery from draining while searching. Do you have a bad signal when it kicks into sleep mode?
I was having a bug with that. For some reason, it would randomly kick into that mode and I couldn't get it out of that. Not sure if it's the best solution, but killing the Settings process made it stop doing that.