The Dude stops recording history graphs

I am running 4b2 and it has been pretty stable for months. Recently it has stopped recording the history graphs intermittently. When I notice that it has stopped - I reboot The Dude server and it starts recording again, then some time later (days) it stops again. All other functions seem to be working correctly, polling, notifications, etc - just an issue with the graphs. The issue seems to be happening more frequently too. I have read in this forum about problems when the database gets too big (2 GB) but my database is only 251 MB. Does anyone else have this problem - have you worked out what causes it or how to fix it?

Are you also experiencing a memory leak?