Hard to say... can be a number of issues... battery might not be sitting properly... try to take it out ...place it, clean connecting metal pieces with eraser ... place back in good.
Can be rogue app crashing your system and rebooting... can be problem with the rom.
Try reflashing your rom.. or another one... see if still have same problems.
If you need help, check the FAQ first for whatever ROM you're on. Then ask questions.