I have seen this behavior occasionally.
I would start by looking for these things:
- intermittent network connections, bad wireless, nic, switch, etc..
- ODBC user(s), reports using an isolation level other than READ-UNCOMMITTED
- antivirus or backup software that is locking records
BTW... have you tried these
- the conversion programs to end labor activity?
- setup verbose logging, see any errors with the same times?
Recently I saw it at a site & never did identify a cause.
Random locks for about a month & then just stopped.
This can be a pain to track down.
Good luck...