Have you performed a factory reset? Doing so is often necessary after a firmware upgrade.
Yes I did. But to no avail.
When you did the reset, did you take out your sd card and test it for a while without reinstalling any applications?
There could be other data there or compatible issue that can continue causing a problem without doing those additional steps.
Yes I did that. Some of the above mentioned problems strated right after the upgrade and reset.
The Marshmallow update for the T-Mobile Samsung Galaxy S5 (G900T) is garbage, if you haven't figured that out by now. I am a recent convert to T-Mobile, and where I am satisfied with T-Mobile, -I am rather unhappy with the Samsung/T-Mobile Marshmallow update to this device. My wife and my brother both have the G900 with two other different carriers, and their handsets operate well, which tells me that some of these issues may be particular to the G900T only.
I had purchased my G900T, new in box, from an Amazon retailer for about $240 a couple of months ago. Since then, I have had multiple issues with this device. Many of the issues that you've described are similar to those that were troubling me. After having this phone do an uncommanded restart while I was on an important phone call with a hospital, I nearly lost my temper and threw this phone at the wall.
The firmware on this phone is obnoxious:
1. Multiple unncommanded resets throughout the course of one day
2. Nuisance messages like the recurring CHECK BATTERY COVER after reboot.
3. Notification Panel Quick Setting buttons resetting after a restart, due to the CHECK BATTERY COVER message.
4. The inability to remove the CHECK BATTERY COVER on Samsung specific devices (THANKS SAMSUNG!).
5. Gobs of Bloatware. I need say no more.
The fact that I'd probably need to mail this phone to Samsung for repairs was unacceptable. Therefore, I've decided to take matters into my own hands. The warranty on this device is now null and void - but it works much better now thanks to some diligent searching online.
DISCLAIMER: (If you decide to root this handset, you're on your own)
I've flashed a recovery called TWRP to my phone using software called ODIN. After doing so, I was able to flash-install a 'rooted' and 'Deodexed' STOCK (but altered) T-Mobile Marshmallow ROM to my phone that I've found online. This device has been SOLID ever since doing such. Granted, there are many things that can go wrong during this process (so make a backup using TWRP before you proceed) but it was worth it. I was, after all, going to destroy this device out of pure anguish. The only problem that I have not overcome is the CHECK BATTERY COVER pop-up. There is software out there that will remove this (XPOSED), but again, Samsung handsets do not respond well to that software (if at all), so I'm still stuck with my QUICK SETTING BUTTONS being reset after each and every reboot. Yup, frustrating. Thanks again SAMSUNG!
After rooting your phone and installing a STABLE ROM, there are a few applications that you may use (under root) to modify the contents of your phone. I've removed several of the Samsung (and other) applications that I never use (Bloatware) that came with this device, thereby freeing up more space in the device memory.
I'm rather content with T-Mobile over my last carrier ($$$), but had I known that the G900T was going to be such a headache, I would have gone with another device. Now - preferably NON-SAMSUNG. After the Samsung Note 7 fires, the inability to remove the device batteries on new Samsung phones, and the frustration that I've experienced with this G900T, I WILL NOT BE PURCHASING ANOTHER SAMSUNG DEVICE IN THE FUTURE.
I expect to get at least one more year of life out of this device before I switch to another device.
Welcome to the wonderful world of having control of your device.