Besides the main problems of constant lag and freezing, here are 3 quests that i cannot complete due to bugs !

1. The Battle for Fort Hraggstad - Join the men attacking Fort Hraggstad

The quest arrow appears on map, but when i get there, no NPC's are present, I tried a few times approaching the arrow, it states quest completed and directs me to attack the fort, but the fort is a ghost town, no soldiers present ? Reloaded a few times, waited god knows how many hours (24-48-72-etc...) same result.

2. Arniel's Endeavor - Place the soul gem into a final convector

The quest states to place the soul gem into 3 convectors, well after the first it tells me to place it into a "final convector", but all the other convectors are bugged, they won't heat up, I tried all the convectors same result, reloaded...same results !

3. Waking Nightmare - Follow Erandur

Once i entered the dungeon, Erandur disappearred ? Reloaded a saved game, but now Erandur seems to be already past the statue door that he is the only one that can open ? The quest arrow appears in the dungeon, but there is no way i can open the main door ! Reloaded a few times and waited in increments again 24-48-72 hours again....same result !

I'm moving this to the spoilers section. Not only can some of the members there assist you with work arounds most of the time but devs do search that area for quest bugs and glitches that need their attention.

The Arniel's Endeavor quest is known to be bugged. See the bug section http://www.uesp.net/wiki/Skyrim:Arniel%27s_Endeavor_%28C%29.

Yea I made the mistake of letting Erander go ingo the temple first, and now I cannot complete the quest.

It went fine on my first playthrough, because I entered before him. Great quest. Too bad its bugged

Yeah, I'm hosed on the Nightmare quest for the same reason. I sure hope this one gets fixed, because I did manage to do it on another character and it was fun.