If you keep increasing voltages and you can't OC higher, then the simple answer is you've either hit an FSB wall (unlikely) or that's as best as you can get out of your components. It might or might not be the cooling, but it might also be the RAM and that's as high as it'll go. Generally speaking, it's usually cooling and RAM that hold you back.....FSB on the mobo next, and CPU last. So that's my guess in order:
Sounds like you might be at the limit. Most 6600's top about about 3.6 +/-, and the BigWater is not exactly a high end cooling system, so you might be topped out. 3.5 or 3.6 sounds right, depending on your RAM. What RAM you got?
For that type of detailed information, perhaps you should speak to Intel. We just know that these walls exist, not the inner machinations of how, where, and why. Chips are different, it happens. Walls happen. Sometimes computers make absolutely no sense, it happens. 3.5 works, 3.6 doesn't...it happens.
If you have a burning desire to know WHY it happens, you need to look elsewhere I think, closer to the source.