SQL Server 2008, database completely locked
Whilst not answering your question re. tracing locks by date range, all sorts of things can have this effect on the database. 

I would first check whether the problem is environmental. Use Windows Perfmon to see what the system resources are doing at the time. For example, I have seen this type of thing occur when SQL Server checkpoint operations takes an overly long time to complete. The aforementioned issue can normally be solved by configuring a "recovery interval" of 1 minute or so.

To be sure of what's going on though, I would setup Windows Perfmon counters to monitor CPU usage, memory usage, and disk queue lengths of any disks on which your DB / Log files may be residing. In my experience (High OLTP) disk queue lengths should be no more than 2 for any disk. You can normally configure a separate server to log these counters to disk every 30 seconds or so in order for the monitoring not to adversely affect your DB server's performance. The number of concurrent connections and locks can also be monitored in this way.