What Hardware version is your router? A1 (says so on the router page)
What Firmware version is currently loaded? Firmware 1.04 (April 2013)
What region are you located? North America
Are you wired or wireless connected to the router? Wired (CAT 6)
Has a Factory Reset been performed? Yes (when downgrading to FW 1.03 and back to FW 1.04)
Was a Factory Reset performed before and after any firmware updates then set up from scratch? Same as above
I factory reset the thing when I thought I was having NAT issues (wasn't really an "issue" but how the router handles UPnP clients differently from other router's i've used).
Again as the router basically works fine for internet connectivity, to what the optimal configuration could be, its a hassle to get the QoS engine to read my correct upload speed (since manual only allows up to 10mb). Before buying the DIR-636L, i used a Netgear WNR3500v2 that was able to manually set QoS uplink to higher than 10mb (It has a slower processor and less ram than the DIR-636L). I switched because IPv6 didn't work on it.
As has been stated before by you, Furrynutz in other threads (like the DIR-655 2mb QoS limitation), the QoS engine may not be capable of more than 10mb uplink (which is [no offence] obviously false if it detects upload speeds higher than 10mb and doesn't show "Not Estimated" or "upload speed has surpassed necessary limits" or what ever it was).
I'm sure a 700mhz top-of-the-line Ralink RT6856 CPU (which is also found in the 826L) could easily handle more than 10mb set manually, but I know it's to get consumers to buy up into higher priced "gaming" router's that have the same hardware, but more flexible firmware.
I have no problem using "automatic uplink speed", but going in after 2-3 days and messing with it for correct values I don't think is the intended behavior of the QoS engine itself. The modem uptime has not reset since the router changes uplink speed, telling me that its more of a router issue (though minor in many people's cases).
Your sticky about Bufferbloat and lag compensation has made me look into it in all of my networking equipment and as I said before, setting up QoS correctly on the DIR-636L does eliminate the latency to where its negligible. Just don't like having to set it "correctly" every 3 days.
I know you are going through the basic steps, but my modem levels are within spec (4 channels at 6 dbmv downstream [it's cold here in minnesota] and 3 upstream channels all at 42 dbmv) and has not randomly disconnected due to power levels.