13" MacBook Pro (Late 2009) - nVidia 8400m stuttering graphics.

Question

Recently, my nVidia 8400M GPU seems to have started acting up on my 13" MacBook Pro (Late 2009).  Whenever it is coming under a lot of heavy graphics i.e. games, the FPS rate begins to drop - understandibly, this is normal. However, it's predictability and duration is not.
Both under Mac OS X Snow Leopard and Windows (via Boot Camp) the problem occurs and will occur under any heavy graphics.  Currently, the only games I play on my MacBook are Source based (Portal 2, L4D2, Team Fortress 2) and Minecraft; all of which have the issue.
The issue is that once the game begins, the FPS is fine, usually hovering around 24-30fps on full resolution with decent settings which is high enough for fluid gameplay.  However, 5-10 minutes into gameplay, the FPS plummets to around 12 fps in a reoccuring pattern (demonstrated as best possible below with '-' being smooth gameplay and and '.' being a 'stutter');
----- . . . ----- . . . ----- . . . ----- . . . ----- (and so on)
It's fairly rapid, the smooth might last for a few seconds, stutter; smooth, stutter, smooth, stutter in a rhymic predictable pattern.  At first, I attributed it to possibly a flaw in the Source code for Mac, it's still fairly new.  But then Minecraft started.  And now it occurs on Windows so it must be a hardware issue.
After maybe 10-15 minutes, it'll resume a normal FPS that's playable (or oddly, if the map is changed or refreshed on TF2, that kick starts normal FPS).  Once it's settled down, you could close the game, come back hours later and wouldn't have the issue - if you put the Mac to sleep or shutdown, it'll occur again on next boot.
Is this a common occurrence with the 8400M on MacBooks?  I used to play TF2 on Windows via Bootcamp before it was released for Mac and can never remember having this issue.
Is this something that could possibly be resolved instore?  My MacBook Pro is still under AppleCare, would it be worth them having a look at it?

Answer 1

You say recently.... I suspect that it has something to do with the 10.6.8 you have posted on the bottom left of your specs. I too had this very same problem and it turned out to be some apps that were not yet optimized for 10.6.8 I had to pop the install disk in and downgrade to 10.6.7... problems have disappeared since then. Good luck! Check your activity monitor and see what the CPU usage is when you boot up also check your system memory... After the update my computer the new MBP i7 2.2ghz was using 50% of the cpu at idle didn't notice it until I finally checked activity after having sluggish performance editing in aperture and restarting multiple times.