Now I have one mining and the other mines for a few minutes then switches to a gazillion gigahash and 100% HW errors
Honestly I've only ever seen this happen for two reasons.
The USB port it is plugged into isn't registered correctly/is going bad or the firmware needed to be reflashed on the chili.
I believe you stated both of these you have verified are not the cause?
Maybe try to hook it alone on a different computer,
try to install a different firmware version,
https://www.dropbox.com/s/zoewijezzhfl3bs/Chili14e.hexThe above firmware might not be the newest but it is the one I'm using on all my chilis and they are running great (Even the ones that did give the 100% hardware errors and one that was dead until I reflashed it with this firmware)
Hope this helps and you get can a stable chili for xmas!
Edit: Maybe try changing the PSU in case it isn't giving it enough juice for some reason? Long shot but I know I had a PSU that was having issues and causes my Chilis to behave a little weird.
I fixed it, don't know with which, but I reflashed to the firmware for units with near perfect engines
I reseated the heatsink to the chips, one of these fixed the problem. its a Carefree mining Christmas!