Nice job!
I am not 330 guy, but for decoding mesages is TSM (CFDS) right way only. If somebody put these mes. numbers into TSM "start troubleshooting" part of Airn@v software, get lot of information and possible causes immediately, includig power sources.Unfortunately my acces is limited to 320 only.
ISIS - not sure with this, but ISIS is connected to IRUs to provide optional heading information (compass scale on bottom of LCD display). If data bus or multiple IRUs fail, ISIS fault can be generated too.
Searching problems in ADR/sensor part of ADIRU system looks better for me.