Suddenly, while lightly using my computer this morning, a bunch of strange colors and shapes appeared. After shutting my computer down (had to hold power button, because the computer froze), and restarting it, the startup screen was covered with yellow dots. After the apple logo appeared in the middle of the screen, it froze again. I shut it off, and unplugged it. After waiting a few minutes, I turned it back on, and started it in verbose mode. It seemed to work, but once I logged in, the computer ran very slowly. I shut it down using the shutdown command, and started it up again, but in AHT mode. When loading, the dots appeared, but it didn't freeze, and after it finished loading AHT mode, the dots went away. A preliminary hardware test suggests that everything is okay, but I started a test in extended testing mode, and after about twenty minutes (still testing memory), the lines came back. The extended testing hasn't completed yet, but the lines have been disappearing and reappearing at seeming random intervals.
There are five lines, about half an inch in width, and they run from top to bottom. They are a transparent yellow color, and sometimes grow/shrink.
The dots only seem to appear when the computer is first starting up, and are also transparent yellow.
This is a late 2009 27-inch iMac, with the latest os. I've had to take it in twice before, once for a broken logicboard (or, it might have been the motherboard), and once for a faulty HDD. It has all the default components, but I've upgraded the ram to 12gb.
I haven't noticed any issues leading up to this, and I wasn't doing anything very graphics intensive when it suddenly started having these issues. Also, if it makes a difference, I always shut it down at night, so it almost certainly hasn't overheated (it's not hot, anyway)
Thanks for taking the time to read this. I plan on calling a local apple store, and hopefully bringing it in to be fixed. I bought this in late 2010, so my warranty has expired, but as it has been serviced twice, I'm hoping that they'll take pity on me, and fix it one more time.
If anybody has some suggestions, or tips, for explaining this issue to the folks at apple, please let me know.