OK I did a full wipe this time, system, sd card. I've got TWRP on there now which works fine. I flashed ICJ then the boot.img as well as wiping the cache.
Phone booted up fine was working for quite a while before it crashed again, the phone was plugged in using the Motorola plug charger when it crashed. I've been using a battery pack too which uses the power connection on the back.
The phone wouldn't turn on until I plugged it into USB for a while (no led) and held down power + volume down. I'm going to charge it up to full via usb in TWRP then see if it still crashes without charging and without the battery pack on.
I hope it doesn't because then it could just be I need to get the HTC plug/cable and stop using the case.
If it's a hardware problem then I'm not sure I'll be able to get it repaired as I bought this second hand (with a receipt, I could see if I can proof of purchase) and have unlocked the bootloader etc.
I suppose if the phone won't turn on though they can't know that.