This weekend my console started to have this problem too. It's a Zephyr, Benq, HDD with 8Gb free, 8 months old. It always worked flawlessly, except for Assassins Creed backup that just don't boot. I don't play games too much so it's pretty preserved.
I moved my console to my brother's and connected HDMI for the first time. Firstly I noticed that the game itself took a bit longer to load (which I can tell by the cartoonish screens in the beginning, there was one more showing). Then he got his hands on it, began a new game and freaked out, riding streets like mad and I noticed that some textures were taking too long to show up, specially the asphalt. Strangely, he was able to travel through the bridge that's blocked by the police, there were no one there. But he did get the six stars.
When he got to the forbidden island, then things got real crazy. Cars were floating above the ground and sometimes there were no ground at all or just a plain blue surface. Buildings looked like cardboard boxes and most elements were just missing. I'm pretty sure that the texture pop-in were never like this until now. After this, any ride to any place was an ugly mess.
Back in my house I tried to clean the media and screwed it. I burnt it again with the same brand (RITEK) and tried it with the same results, even cleaning the HDD cache (the maintenance thing). I could reproduce the "unblocked bridge" issue. I've read somewhere that other games were also having the same problem (Oblivion and Mass Effect), and it's the Benq laser beam that's physically damaged, for my despair.
Yesterday I tried my own backup of Oblivion, and it worked ok, with barely noticeable or tolerable pop-ins. Then I immediately switched to GTA again, and instead to keep driving, I started to accomplish the next mission. For my surprise, there were no bad pop-ins at all, even though it's taking a bit longer in the first load. I kept messing around and everything was suddenly fine. Today I drove a fast car throughout the first island and didn't notice delayed textures.
The problem seems to be magically fixed for now...