I have a Samsung Galaxy S3 (i9300) that I've been using for slightly over a year. I am a rather heavy-duty user of the phone, and have used in intensively for that time. I do take care of the phone though, so I doubt if physical damage was a cause of my problems.
A few months ago, I moved from Cyanogenmod 11 Nightlies to CM11 Snapshot releases. When I updated to CM11 M6 a few weeks ago, I started noticing dramatically reduced battery life. Not immediately, but in the days following the update I noticed my battery would drain from full to emtpy in about 5 hours, even if I left the phone unused. I turned off all non-essential features on Android, don't run apps in the background, et cetera.
A week or so ago, I started noticing strange behavior. The phone would freeze up, and then spontaneously turn off. If I rebooted, it would sometimes reboot successfully, other times it would freeze during boot. I did a wipe and reinstall of CM11 M6, but the problem persisted. I noticed, though, that whenever the phone did this and I did manage to restart, the battery would be about 30% lower than it was before the crash. I also noticed that a few seconds before every crash, my screen would start to flicker, as if certain horizontal lines suddenly had their brightness levels fluctuated. However, if the phone did refuse to boot up and froze during boot, if connected to a power source by the charging cable, would be fine. I've also never had the phone freeze while charging.
I suspect that my phone's battery might be going. Am I right in this suspicion, or am I missing something?