hashcat stops after about 37 minutes

[patpro]

Some precision: usually I can make long sessions of hashcat without any problem (several weeks), but I don't usually use -6 mode.
A quick & dirty log analysis shows this timing:

Code:

37.9167
38.8333
37.9167
42.8
37.8167
37.9833
38.0667
38.0833
38.0667
38.5333

No error, nothing special (neither in hashcat.log nor in syslog/dmesg/...)