I had a problematic Nexus 5 - it constantly complains that [some app] has stopped (usually Google Play Services, but sometimes gapps or hangouts). It's usually Google Apps that crash.
I downgraded to Android 4.3, which helped a bit, but after a while started crashing randomly. After some thought I decided to try a fresh install. I installed stock Android 6, did a factory reset and cache wipe.
And it still complains all the time. The phone is goddamn unusable. What can I do???