On all of my sites (and one more often than the others), I get crashed sites due to specific database tables seizing up at least once a week.

75% of the time it is the table sessions
25% of the time it is the table whos_online

It is becoming enough of an issue that I can no longer overlook it.

Is this the type of thing that can be traced back to a source, or are there too many things acting on those tables to figure out the cause?