I had the exact same problems, the screen flashes and stutters, network signal is lost and the phone turns off, it is due to uncalibrated battery.
There's a simple fix, download battery calibration from play stor (green android icon with percentage), charge your phone to 100% without using it, then click battery calibr. inside that app, then doscharge it till 0% (do your normal usage as ussual) and then charge it back to 100%
For healthy battery life don't let it drain lower than 10% it's the li-ion physics and always charge it fully, don't charge from 0 to 30-50-60% because the ions in the battery will not have a good flow.
Everything has it's lifespan but if it's configured and used properly it can have a long lifespan
Also to mention, if my recommendation doesn't work, your battery could be swolen due to inproper charging / heavy oc-ing or your charger is messed up.
Also to say, that flickering occured to me when i flashed a new rom and when i do lot of reboots, then the batterystats freak out, the voltages also and also your phone.
Charhe and use it properly, try to stay with a rom at least a couple of days to see how it'll go.
Sent from my pebble blue scoobie doo using Tapatalk.
PHONE: Samsung Galaxy SIII i9300 16gb pebble blue
KERNEL: Boeffla kernel 5.3 stable
ROM: Stock, debloated, fine tuned XXUGND5.
When life gives you lemons, make a lemonade.