No shader effects for cloaked enemies on newer gfx cards?
Well then this problem can't be solved that way. But thanks for trying to help! HERE IS THE FIX!! 
Someone found out how to fix this some time ago. The trick is to fool the game into thinking you play on ATI ant the game tries to apply some "fixes" because of that and the game will work. It's all because the engine of Halo doesn't scale so well on newer graphics cards. In fact halo uses specific configs for every graphics card (it determines which fix to use for example by reading vendor and driver id and comparing to a config etc.). But it also might do some other stuff in the background if it "thinks" you have a specific card (hardcoded). Still, it will work if you use the following method:
http://gbxforums.gearboxsoftware.com/sh ... hp?t=60814
Other thread about this:
http://gbxforums.gearboxsoftware.com/sh ... hp?t=60536
Someone found out how to fix this some time ago. The trick is to fool the game into thinking you play on ATI ant the game tries to apply some "fixes" because of that and the game will work. It's all because the engine of Halo doesn't scale so well on newer graphics cards. In fact halo uses specific configs for every graphics card (it determines which fix to use for example by reading vendor and driver id and comparing to a config etc.). But it also might do some other stuff in the background if it "thinks" you have a specific card (hardcoded). Still, it will work if you use the following method:http://gbxforums.gearboxsoftware.com/sh ... hp?t=60814
Other thread about this:
http://gbxforums.gearboxsoftware.com/sh ... hp?t=60536