Hi,
I've been on Merlin firmware for 3/4 years on my A68U and have always been very happy.
The device is running as a wireless router, with AiProtection (Parental Control / Time Scheduling, DNSFilter), firewall on, a bunch of IP adresses for my connected devices (wired & wireless), and one OpenVPN server from time to time. Nothing fancy really (no QoS, no IPTV, ...).
Everything has always been fine except the weird symptoms as the ones mentioned in this thread (don't remember when it started, maybe from day 1, I really can't tell anymore), on several devices: android mostly but also linux. It felt a lot to me like something was wrong with DNS. But nothing really stood out in logs. Another hicup I had was that the web interface was often very slow on some devices (android and iOS mostly, not linux). I applied official versions one after another up until 380.69, hoping that my issues would be fixed at some point, but unfortunately, they never did. So I concluded something was off in my network and stopped blaming the A68U or Merlin firmware for it.
And then AiMesh got my attention (I actually have two A68Us, the other one is mostly off, configured as a media bridge). As I had a few hours ahead, I took the plunge on my main router (clean install) hoping my issues would go away in the process. Nope, still same problems. After reading comments about AiMesh not being quite there yet, I switched back to a clean Merlin 380.69. Nope, same issues. Heck, I can give a shot at those alphas / betas out there. I tried all versions available at the time and the miracle happened with one of them: 382.3_alpha1-gb26683d. I don't know how or why, or it may just be a coincidence or something. But this version has been working flawlessly for a whole week. No more connection issues, web interface comes up instantly on all my devices. I also use the "Asus Router" app on android and iOS to grant/deny access to my children's devices. Everything works perfectly and is buttery smooth. For those wondering, there have been 0 changes in my network.
Hope this can help some of you guys out there (and that someone will be able to track down the issue).
tl;dr
I used to have such symptoms on my A68U. All fixed since I applied 382.3_alpha1-gb26683d.