I once was a bit bored a day, and my 360 decided to be a fucking pain in the ass, too - by locking up while playing some R6 Vegas [online] a few times. So, having the previous experiences with [computer] hardware, I had the astonishing idea to get rid of my consoles warranty once and for all.
Everything done at your own risk! No guarantees given. Enough of the legal bullshit. This stuff can help your box. ;)
Here's the plan:
#1 Get rid of 360 warranty sticker.
#2 Put sticker on female's ass (preferably, but any other ass is fine I guess).
#3 Get a hold of VGA chip/memory cooling heatsinks, and if you want to take it one step further get some Talismoon Whisper Fans.
#4 Open up your Xbox360 following the tutorial of choice (use Google you lazy fuck).
#5 Drink your milk:
Right...continued.
#6 The actual mod starts now. Put your 360 in front of you like this:
#7 I'll transfer that to this sketch now, as I didn't think of taking pictures back then. Pretty much self explanatory I guess.
#8 Now the magic begins. We place our VGA chip/memory heatsinks first on the visible chips in that sketch, and then we stick them to our GPU heatsink sideways increasing its surface - which will (if physics apply) result in better cooling thanks to more place for the head to spread.
#9 If you got some heatsinks left, or bought too many for whatever reason, you can further increase the cooling performance by stacking shit - literally. Place one heatsink on top of each other but be careful as there still has to go a disc-drive on top!
#10 Now exchange the old, loud-as-hell original fans with the flashy (thanks to LED's) new Talismoon fans, following the packaged instructions.
#10 Re-assemble box.
My result: No lock up since then
ever again. Tested playing Vegas, The Darkness, and Overlord straight 12
(!!) hours.
Yours may differ, let's hope for the best.
Furthermore, I can only recommend doing the additional rear fan replacement. You can get these pretty cheap, I got mine for around 20 bucks on eBay.