I just located the source of this problem. My ISP PowerDNN was optimizing my site without my knowledge or permission. This optimization included randomly truncating the sitelog table, even though I had setup DNN to keep 60 days of site log.  They now have remvoed my site from the list of sites to optimize!

For people who need to closely manage the size of their DNN database there are a couple easy things to do:

1. Set the number of days of site log to retain down to a reasonable number. If you are like many sites who never look at the site log 0 might be ok. In my case I have a process that analyzies the sitelog and looks to see which of my customers information has been viewed and how often, so I want 30-60 days. The limit of sitelog is on Host Settings (Adv)  page and on each portal's Site Settings (Adv) page. The site seting can't be larger than the host setting. Why PowerDNn didn't use this method is beyond me.
2. The other log that can eat up space is the EventLog. See Log Viewer in the Admin menu.  The Edit Log Settings at the bottom right allows you to set limits for each type of entry.  I severly limit these for normal operation. When I an troubleshooting a specific problem, I temporally up the ones that I need more data on. The out-of-the-box DNN defaults are quite large for the event log and could be set to lower values at most sites. I suspect that the DNN developers have these set high because while developing they get lost of exceptions.  DNN Creative has a great video on how to manages these values.

I followed some instructions recently to speed up DNN, one of them was to have the log go to files and not the database.  I didn't think about what this might mean, such as not way to analyze the SiteLog.  I have now gone back to loggin in the database.  You might want to check the host seettings.