Tab S keeps turning itself off & on

Since you already tried a Factory Reset you might also want to wipe the system cache. In order to do this you need to restart your Tab into its Recovery Mode. Note that while in Recovery Mode it's a text-only interface so you need to use the indicated buttons to navigate through the various options. Select the 'wipe cache partition' option (this itself should only take a few seconds) and then just restart normally. Instructions on entering Recovery Mode are shown here:
http://www.hardreset.info/devices/samsung/samsung-t800-galaxy-tab-s-105-wifi/recovery-mode/
But this is only a 'maybe' kind of suggestion. If it does help that's good but if not you might need to give some thought into re-flashing the stock ROM. Flashing a ROM is basically the same as reloading the operating system on a computer. (A Factory Reset does not do this, all a FR does is wipe the data partition clean, it has no effect on the installed operating system.) Also, these are just software solutions, hopefully your Tab isn't having any kind of hardware-related problem. Anyway, try wiping the system cache and if that doesn't make any difference, post the exact model I.D. of your Tab S here so we can make more relevant suggestions.

You might also want to try using Samsung's Smart Switch utility to see if it will update anything on your Tab S. Install the utility onto a computer, connect your Tab, and see if it does anything.
https://www.samsung.com/us/smart-switch/
https://www.sammobile.com/samsung-smart-switch/

If you're still having problems, than you might want to try flashing a stock ROM.
This also will require a computer and the following applies to a Windows PC. If you used Smart Switch, during its install process it will have installed the necessary Samsung driver so you don't need to install it on your own.
Now download and install vers. 3.11.1 of the Odin utility from here:
http://www.droidviews.com/download-odin-tool-for-samsung-galaxy-devices-all-versions/
You'll also need the ROM itself. Go here and download the ROM that matches your locale and your carrier:
https://updato.com/firmware-archive-select-model?q=SM-T800&exact=1
Once you've got all this downloaded and dump everything in a new folder so you know where all the parts are.
-- Extract (unzip) the downloaded firmware file
-- Start up the Odin utility (note that Smart Switch and Odin may conflict with each other as they both will compete for the same background system resources. Go into the Windows Task Manager and be sure that the Smart Switch process is not running. If so just kill it.)
-- Boot your Tab into its Download Mode (simultaneously press and hold down the Volume Down, the Home, and the Power buttons):
http://www.hardreset.info/devices/samsung/samsung-t800-galaxy-tab-s-105-wifi/download-mode/
-- Connect your Tab to your PC and wait until you get a blue sign in Odin
-- Add the firmware file to AP / PD
-- Make sure re-partition is NOT ticked
-- Click the start button, let the utility transfer the ROM onto your Tab. Might take a while, it's copying over 1GB+ of data.
-- When it's done, you should see a green Pass message. Disconnect your Tab and let it reboot.

Just joined this forum because I have the same problem with my Tab S.
Thx to SVIM for his advice I will try out and inform thr results later.