Intermittent alarm clock issue

[AC Question]

After the latest OS upgrade to Marshmallow 6.0.1, my alarm clock app has started failing to go off...intermittently. At first I thought it may be a compatibility issue with the new OS, as I had been using the AVG Alarm Clock Xtreme app for the last 2 years. I switched to the stock alarm app, but the same thing happens with that app. No rhyme or reason to it. It will work correctly for several days, then one day it won't sound. The apps apparently don't wake the phone, as I will wake up naturally, wonder why it's light outside and manually wake the phone to check the time, at which point the alarm starts sounding. Same random failures noted in both the timer and alarm clock functions of both apps. I've uninstalled and reinstalled the AVG app, restarted the phone several times - including a full reset, and had it both charging and unplugged. Fail rate is still random. Samsung Galaxy S5/Verizon. Similar thread here: http://forums.androidcentral.com/showthread.php?t=482305&mn_qa=1 but no helpful solutions (plus the thread's over a year old, so maybe a fresh set of eyes looking at it will help?)