After zoning into Subterran I've seen the client simply become unresponse while waiting for the rest of the party several times and someone will disconnect. That hit me twice on reavers fate last week too. Starting to toss a few buffs and the system stops responding. Last DQ I did the party leader DC'ed when we were working on efreet right after entereing and there was some choppiness. I do not disconnect and the client becomes responsive again in a few moments.
Running thru the desert 2 of us could not proceed into the undead section because we would hit it and bounce back to where we hit it. Someone said try to go in backwards and that worked. Not sure if that was a weird voodoo fix or not.
When I log in and out swapping inventory or checking timers I see alot of lag while the system is populating my hotbars etc. The more I do it the worse it gets and frame rate drops followed by choppiness.
If I play for a while running thru several quests it seems to get worse and requires a restart.
I do not have the precise dates and times, or party composition. I'll watch for it now for the next time it happens, but hopefully that helps a bit. Around 4AM EST the lag gets worse almost every night.
EDIT: The optional at the end of Weapons Shipment.
I run that frequently (my idea of frequent is 2-3 times per week) because I still haven't gotten a mad lute and I want it in my weapon collection. The end fight with quicken on sudden my healing spells on capped spellsinger and my buddy on a capped AoV will simply disappear. No incoming healing from either of us. The spell is simply lost and never hits. Usually we're running it on hard, on Ghallanda, around midnight 1am ish ETA.
The typical method we use on the optional is to charm the tieflings above the the first portal so they can draw devil aggro ranging them while we beat down the portal, ddoor after the portal is down, buff up and invis sprint past the remaining portals to the end fight. (EDIT2: AoV aura certainly made the invisible sprint more complicated.)