I've used various build of dd-wrt on my Asus RT-N16, but they were never very stable - possibly because the NVRAM was very near full.
I 'bit the bullet' the other day, crossed my fingers (and legs!) and flashed:
Tomato Firmware v1.28.9054 MIPSR2-beta K26 USB vpn3.6 (file name: tomato-K26USB-1.28.9054MIPSR2-beta-vpn3.6.trx)
So far the router has been very stable and I note that only 38% of NVRAM is in use. I'm still feeling my round Tomato and there are one or two points I need to sort out - but I'll raise those in other threads.
Many thanks to those who've supplied and support Tomato.