Ok, here's my problem.
Ever since I downloaded the DLC's (I've got all 3 of them), my frame rate has gone down. I find that if I turn off the console and let it cool off, it works for a while after that. But after a few hours, the game gets glitchy and, for lack of a better term, "choppy" after that. I'm sure we've all run into this type of problem before, so I ask you: is this due to the DLC's or overheating...or both?
It could be anything. The general consensus is that once your game save hits 10MB or higher things start to get annoying. I have all 3 DLCs and a 16MB game save. After a few hours on straight playing I get unplayable amounts of Lag and framerate issues. And I basically do what you do. Quit game, let the system set a minute, and go back to playing.
I don't recall have framerate problems with Dawnguard, but Dragonborn was terrible. Especially Tel Mythrin. Most freezes I've had lately were in that spot on the map. It's annoying. And this is on the 360.