So I went to play some GTAIV last night and there was significant pop-in, missing textures, and an overall slowness. I turned it off and went to play around in the marketplace and everything was slower and choppier than normal.

Has anyone experienced this or any ideas what I should do?

never happened to me, perhaps your system is overheating?

Happens to me all the time. No missing textures or artifacts but the game is visibly choppy and jerky. I definitely have a defective 360 though. My first one was great until it RROD and MSFT sent me this crappy thing.

I haven't sent it back because it fixes itself after I reboot it.

Ive had this a few times while playing COD4. I thought it was lag, but I could hear my 360 literally struggling to keep up and I had 3/4green bars in the game. Very choppy/jerky. Thankfully it rarely happens. Its never happened on another game though..

This is almost certainly an overheating issue. When a processor's temp gets up, it becomes much less efficient, directly correlating with a drop in fps, etc (video). If you keep your console in cabinet/entertainment set, especially one with a closed back on it, consider knocking out the back, relocating the system, or getting one of those laptop cooler things to set it on (or this thing, specifically for the 360.

Also, see if the vents need cleaning. 3 years of dust can suck for hardware.

The more developers push the boundaries of available hardware, the more air-cooling just isn't going to cut it. I wouldn't be surprised if at least one of the nextgen consoles was watercooled.