Interesting issue... I just built an encoder with similar parts and have had great results for my church. We've streamed 24/7 for about 2 weeks now...
Here's my setup:
Asus P8Z77-M motherboard
Core i7-3770K CPU
Samsung Pro 840 SSD 256GB (support told me SSD wasn't necessary)
16GB DDR3 1600Mhz Kingston RAM (2x8GB)
Windows 7 Pro SP1 64bit
BlackMagic Decklink Studio (PCI-E x1) Card
1080i content coming in via HD-SDI with embedded audio
So, same CPU, possibly same SSD with a different capacity... Like you I only hit about 30% with all 4 going (HD, High, Medium, Mobile) at once. I don't get any errors though.
If yours is saying the CPU can't encode fast enough, I would start by ensuring you have the latest chipset drivers installed for your motherboard. Intel's IvyBridge setups will work natively with Windows 7, but the updated drivers do wonders for performance. You can get the latest from Intel's website, or possibly from the motherboard's manufacturer's site.
You should also ensure the drivers/software are fully updated for the Intensity Pro card if that's your ingest point. I'm not familiar with that product but have had great success with the Decklink cards (though they are touchy in various motherboards, which is why I ended up going with the Asus listed above as I know it works).
Also, what TYPE of internet is that? 50d/10u is plenty, but is it a fiber or cable modem of some kind? The error isn't indicating the bandwidth is a problem, but bandwidth QUALITY matters too.
Are you recording to the local drive or have you unchecked that option in the settings? I don't record because I have no need to... but perhaps this is causing an issue for you?
Lastly, did you install any other software like an anti-virus or firewall of some kind? Anything else that could be running spikes on the CPU?