Problem! Samsung Galaxy Tab S 8.4 restarts. Keyboard takes 0.5 h to appear after "On". "Solved"

Work around: keep it on, in power saving mode if needed.
To get it on when it restarts continuously I hold in Power Button 5 second until I get "Restart", select Restart. If it does not switch on I try again.
Keyboard takes 0.5 h to appear after "on". Haven't seen someone else reporting this.
"Process system isn't responding" - Wait or Close options doesn't make any difference as far as I have noted.
Problems came with Android upgrade from 4 to 5.0.2 (required by my bank.) Tablet is "clean", I haven't fiddled with it, like rooting....
Tried re soldering battery contacts. Checked battery plug/socket, looks OK. Deleted some apps. Disabled some apps. Only some 15 running. Didn't help.
Available memory 3 GB.
Processor runs hot, normal? Battery changed half a year ago.
Samsung Galaxy Tab S 8.4. Model number SM-T705
Upgrade to Android 6 available - but I don't try - can lead to unsolvable problems.
Work done on the road in hotel room. Borrowed solder iron, solder, magnifying glass from hotel. Steadied the solder iron with a second hand via a metal wire - very delicate terminals at battery plug only 1/10 inch, 2 mm, apart. If you hit some component you may kill it.
Amazing that these things don''t work, and they are still in business. I have invented, designed, made and sold some things that worked. Tactical compasses for sail racing: https://sites.google.com/site/compassansar . Stand Inside 70 MPH, 120 km/h, Wing Skate Sails: https://sites.google.com/site/icewinghomepage/ . My Home page: https://sites.google.com/site/andersansarshomepage/