BTW it's normal that you don't see any files in /media/ptgui_tmp. PTGui
creates a temporary file but immediately unlinks it. This is so that no
stale temporary files remain if PTGui crashes.
if you do:
df /media/ptgui_tmp
the free space should decrease once more temporary files are generated.
Could you check if this is the case?
What do you have in Options - Advanced - Max RAM? Could you try setting
this to half your available ram (65000 MB)?
Also it might help if you add some more swap space (say 64 GB). On linux
the out-of-memory killer can be aggressive under memory pressure. I
assume PTGui is killed by the OOM killer, but please confirm this by
checking dmesg.