Check your apps - may be an incompatibility. If you're rooted, make a backup, then perform a factory reset and log in to your Google account. See if the phone is usable barebones or not...
Then, start adding your apps one by one and running for a day between app addition to see if you can pinpoint which app it is.
If it is unstable after a factory reset with no apps installed, then maybe the update did not take very well - or maybe your rooting didn't take....