Since my disappointment at latest update failing to fix my problems ( multiple restarts and crashes, generally sluggish etc ) I have been trying various things, especially since realising many have same problems ongoing and repair centres all saying phones fine it's a software issue.Since removing my Google account and setting it up again
( settings > Accounts and backup > manage accounts )
Mine is like a new phone! Fast and responsive again and refusing to crash and restart when trying to recreate known situations that trigger it.
I will keep this updated in case, but at least it's worth a try if like me you were about ready to throw the phone away in frustration and fed up with generic fix advice, clear cache etc and simple to do.