seldom crashes on Dell E6330 with 12-CURRENT

Hello,

I own since July 15 a new laptop and faced 3 crashes, details see below.
What can I do to gather more information? Thanks


notes in general:
- hardware: Dell E6330, amd64, 4 cores i5-3360M, 8 GByte RAM, 250 GByte SSD w/ TRIM
- WLAN: urtwn0: MAC/BB RTL8188CUS, RF 6052 1T1R
- kernel: GENERIC r302904 (July 15)
- poudriere 3.2-pre: compiling ~1800 ports in 4 builders
- swap as plain file in /usr/swap01, 8 GByte
- apart of the crashes below, no crash during buildworld, buildkernel and
  ~48 hours of poudriere fetching distfiles and making ~1800 ports
- no errors on 'memtest 1G'

crashes/observations:

16.07.2016 07:12 
- on shutdown swap device (plain file in /usr) could not be detached
- drop to kdb
- nothing in /var/log/messages

17.07.2016 19:41
- hard locked, had to power-off
- last command issued on console (no X11): cp -p * /usr/PKGDIR (around 1700 files, 2 GByte)
- nothing in /var/log/messages

20.07.2016 19:27
- hard locked, had to power-off
- last command issued in xterm: fgrep mra... ~/*
- nothing in /var/log/messages

-- 
Matthias Apitz, ✉ guru_at_unixarea.de, ⌂ http://www.unixarea.de/  ☎ +49-176-38902045
"Wer übersieht, dass wir uns den anderen weggenommen haben und sie uns wiederhaben wollen,
kann von den Kämpfen der letzten Tage keinen verstehen. Und kann natürlich auch keinen
dieser Kämpfe bestehen." Hermann Kant in jW 1.10.1989