Any strange battery behavior? You might want to try a new or different battery.
Corrupt SD cards can also cause odd behavior. Try unmounting your card (in
Settings>Storage), removing it, and rebooting.
Some glitchy app could be causing instability. You could boot into Safe Mode, which prevents all 3rd party apps from initially opening. If it seems stable, then start opening apps one by one (slowly), and see if the crashes recur.
Another thing to try is a cache partition wipe, which won't erase any personal data. Steps for the cache wipe and Safe Mode are here:
Reset: Samsung Galaxy S 4 | T-Mobile Support