The table in the database which stores which of the latest threads have been read has recently been changed to a HEAP memory structure, which means that it's now completely stored in RAM. This was done because, as you can imagine, the table gets written to every time a member views a thread, so there is a speed improvement there, as the old MyISAM table format succumbs to table locking (meaning the entire table locks up when it's being changed, so multiple writes must wait in line).

However, I believe that the database server was restarted once or twice over the past couple of days, and when that happens, all of its HEAP tables get wiped out.

Let's gie it a couple more days as I'm not sure if the problem is the HEAP structure to begin with, or if it's restarting the db that caused the problems.