I am having the same issue but it seems, alot worse than you guys.
I got mine from o2 on Friday, updated to the latest BTU rom (JULY from sammobile) and by Saturday then phone kept locking up or was turned off in my pocket. Now 7/10 times I try and wake the screen up with either the power button or home button the screen is blank. Now it is even worse, i get a white noise screen and cant see anything, phone seems to be working but will crash pretty quickly. The screen has the white noise on it as soon as I turn it on and it wont go away, although I managed to last night but its happened again.
Is this a hardware fault? Or is it software? i have tried disabling the SHealth app and the Remote and that has made no difference, I have disabled a lot of Samsung apps too and its still the same!
The only reason i got the phone is because my girlfriends previous phone died so i paid for the upgrade so she could have my S4, which was flawless. o2 are going to make me wait upto 5 days till i get another phone if i send it back to them but i am worried i will just get another one back with the same issue
Any ideas? Is there another ROM i can try on my phone? If i can get the screen to come on that is.