Strange bootloops.

So hey.
I'm facing a really weird issue with my phone. I currently use DU and it works perfectly, however whenever I install 'Ubermallow' or 'Omni ROM' or 'chroma' or even PN for that matter they all initially work perfectly. I go ahead and install AK and xposed it's all still fine, then I randomly restart my device. It starts bootlooping. So I think maybe it's substratum going haywire I remove the overlay, and it's still looping. I end up rm-rF'ing the vendor partition and reflash that to no avail. Now I'm thinking AK kernel is messing up, but it works fine on DU. Xposed could also be it but again I've used it for so long on DU and before that on PA. So if anyone has any advice or anything drop a comment. Thank you

Trying to downgrade MM to LL and install 5.1.1 based ROM - getting bootloops

Was on rooted stock 6.0.1 and wasn't able to get Xposed working, so I decided I want to go back to 5.1.1. No matter what path I try, I can't get anything except unrooted stock 5.1.1 to boot up.

Here's what I tried
Flash ODIN 5.1.1 DOK2 image > boots up fine
Flashed CF auto root - does not boot up. Keeps rebooting
Wipe everything in TWRP (cache,data, dalvik) > keeps rebooting
Flashed ODIN 5.1.1 DOK2 again > boots up fine
Flashed TWRP in ODIN > can get to TWRP and made nandroid backup
Installed 5.1.1 ROM (TEKXnesis) > does not boot up. Keeps rebooting

What am I doing wrong? Do I need to flash Kernel? Modem? Etc?