I am currently using RT-AC68U_3.0.0.4_374.40_alpha4 Merlin's build but the 2.4ghz band seems to flake out to 5mbs randomly. Should i change to the beta 3?
I've had good stability/performance with Merlin's 374.39. Just have to make sure your security settings make sense (e.g. don't enable anonymous ftp access over wan).
I've been seeing posts about random reboots with 40 (and the Asus firmwares it's based on), but it's hard to know how common they really are because people post when they have problems (and most don't post if everything works fine for them).