[Q] Extremely wierd SGS3 Problem

Okay first of all, I've scoured not only XDA but even Google for an answer to this problem to no avail. So if anyone has/knows of any thread that has the answer, please kindly reply or notify me so we can have this thread removed.

Here goes,

I'm experiencing a very unique (afaik) problem with my Galaxy S3. This started a few months back, when the phone started getting random reboots/ freezes and terrible battery life on ANY ROM/Kernel. I've tried CM, SlimBean/Kat, and even Stock and nothing worked. Finally I figured maybe the battery was shot. So a few days ago I went and got a new battery, and that's when it got weird.

I used the latest SlimKat after replacing the battery and after powering on the device, EVERYTHING seems perfect. It's really fast and responsive, battery drain while screen is on is extremely acceptable, things worked like new again. UNTIL I switched off the display, after 5 mins, I pulled out the phone and it was switched off. At first I thought maybe the initial charge wore down so I charged the phone to full, then powered it on. Again, everything went smoothly BUT, after switching off the display for more than 1 minute, the phone shuts off automatically.

I switched to the latest stock 4.3 ROM and restored everything to factory settings and it seems to have helped at first. As usual, everything is fast and battery doesn't drain so much BUT, after putting the phone to sleep, it takes 5-10 seconds for the screen to wake after pressing the power button, and even if it wakes up, it becomes EXTREMELY laggy and often freezes requiring a reboot. As usual, after a fresh reboot, everything becomes awesome again until I have to turn the display off for more than a minute.

I've tried everything, even installing an app that prevents deep sleep but the problem persists. It improved a little after flashing stock rom and reverting everything back so I'm really hoping this isn't a hardware issue. Is there maybe something else I can try, settings/software-wise? The phone is almost 2 years old and is already out of warranty.

I'm hoping it's not the sleep of death thing because the phone isn't exactly bricked. I can still use it for as long as it doesnt have to go into deep sleep (I turn it off before putting in my pocket and turn it on again when I need to browse/use the camera which is all it's good for in its current state.)

Also, the symptoms don't really fit the usual "Sleep of Death/Brickbug" Symptoms I've read about.

Already tried that. I did full wipes/formats on all my installs. I even tried not having an external SD inserted for the Odin installs. I tried not installing any app on the phone and it still gets messed up after going to sleep. As fresh installed AOSP or Pure Stock ROM. Odin flashes no problem so I'm really hoping nothing hardware-wise is messed up. I had to re-flash PIT after getting a new battery though, could I have messed something up there that would lead to this?

I did some minor undervolting way back before JB came out but not since over half a year ago. I did the eMMc check and yes, I do have the inane chip.

Just an update, I re-flashed a PIT file and a rescue firmware and it seems to have helped a bit. It was running fine for over 12 hours aside from a random reboot just now.

I'm observing the phone right now to see wether the reboot was just coincidental. I really wish this is just a software thing.

Okay one more update, 2 days in and I've only experienced the one random reboot. It's still sometimes slow when waking up, but something around 2-5 secs sore not 5-15 seconds slow. I'm thinking that must be due to touchwiz bloat.