So I've been having a bit of trouble with the Google Nexus 5 model when adding custom roms. At first I was unable to make backups of my stock rom, but then when that problem was solved I installed Cataclysm 6.0.0 Hammerhead Marshmellow. Worked fine at first, install was brilliant, messed around with a few of the settings and customised the LED and was happy with the result. Then I rebooted my phone and as it loaded up just past the android logo it came up with an infinite loop of "Unfortunately the System UI has stopped" with an option to press ok, which if done brings up just another box saying the same thing and goes on forever. It wouldn't load up at all and so I tried to go into recovery and access my backup of the stock rom but then I got some error about the MD5 mismatch and wasn't able to restore from my backup. I tried a few solutions including clearing the cache and also a factory reset which somebody suggested but after the reset it reinstalled itself again on load up and then the same problem occurs. Is there any way I can put another ROM on without having to load up the phone fully or anything I can do to solve the issue? I think it might be to do with marshmellow because my phone had issues installing the official marshmellow update in the first place. Thank you in advance for any help offered, I can't use my phone until this is fixed so it's very much appreciated.