High memory usage

Hi folks I have a real frustrating problem with high memory use. The router is running Kongs latest DDWRT release (15-3-18) but this problem occurred with earlier releases too. Ever since installing Kongs first release i have religiously backed up and restored (when needed) my config after every update of his. I have setup 4 WiFi vlans (one of which is configured to use OpenVPN), I have enabled vlan on one ethernet port. I no longer run any samba, or usb hdd although I did try it along time ago. Upshot is the router MUST be rebooted every 24 hours or the memory fills up and router accepts no more connections. Does this sound normal ? How would I go about diagnosis ? I have read that once memory and cache fills up it will just flush and reuse as required but does not seem to do that for me.. advice from the group would be appreciated thanks

Thanks for the reply. Could never get the functionality I required from stock hence the move to DDWRT. Also as I've been so happy with Kongs builds I've never tried Voxel either.