There appears to be a pretty nasty memory bug when using DD-WRT build 14896 on a TP-Link WR1043ND router. From the Cacti graphs below it clearly shows a sudden drop off in available memory, then after a reboot things get cleared up again.
Seems others have had similar issues.
I intend to try the BrainSlayer r20675 release to see if that resolves the issue – and will report back.
Confirmed the BrainSlayer r20675 release firmware did the trick! Here are some new graphs showing no downtime with no complete memory drop-outs over 2 days with very high numbers of connected clients: