Writing An Intelligent Character

I'm having trouble writing an intelligent character. He seems to come off... pretentious and that's not what I'm going for.

I once interviewed a prof from UBC about a space telescope. He recently was made OC and is probably one of the smartest people in Canada. In fact, I heard that he's included in the special features of the new Stargate DVD explaining real life science behind the story. He's a good choice for that because during our conversation, although his IQ is probably through the roof, he never once made me feel stupid or that I was out of my league. He explained everything to me in terms that I could understand with such wonder in his voice that it made me want to take some of his classes at UBC. He was completely unpretentious and generous with his time and nice and highly intelligent. We went off on a tangent about life out there and his thoughts on it and talked sci fi tv and I thought he sounded like Phlox a little bit :P Just an all around nice guy who happens to have more brains than me and both of my brothers combined (and we're all pretty smart people). He was well adjusted (not socially stupid like so many other geniuses are portrayed), talkative, relatable and friendly.

I want my character to be like that prof. But, he's coming off more like Rodney McKay. It's a fine line... this character is highly intelligent and if he's not careful, he can make others feel stupid without really trying... just by explaining. I might take this flaw and make it a bit comical, but for the most part, I'd like to have him be less Rodney and more space telescope guy. I'm not sure how to do it... I don't want to take away from his intelligence and that's what it feels like I'm doing when I change his dialogue. He is really smart and I want that to show without him seeming high class, high strung, high attitude or God like or something stupid like that.

For the life of me, I can't think of an easy going, nice, jovial, genius character to use as an example and look to for advice. Any suggestions would be welcome.