My dad is one of these sorts of folks. He is very well-read, and very well-educated, but he does not know everything (although don't you dare try to tell him that, yeesh!). I used to be like that too, but realized I sounded just as much of a jerk as my dad did when he would pontificate and drone about stuff, so I worked really hard to stop. It still pops up, but that is rare, and I usually just bean-dip myself to stop
The best one I can recall is when Dad actually argued with me over what my job pays. I refused to tell him, and he kept going on and on until I said, "No, Dad, no one makes that much in this field unless they *own* the friggin' company! I'm low-man on the totem pole; I only make $$ a year." He told me I was wrong! I asked him how he knew, and he claims he spoke to so-and-so, and looked at this site on the internet, and I asked to either hear from so-and-so, or see this site he went to. Couldn't produce, and it was suddenly unimportant. How about that?
We're having to go through the "know-it-all" phase with my Mom, who also is under the impression that she's a doctor >_<. Ugh. But she thinks I'm a lawyer too, and has told people that, so why should I be surprised? Luckily, she's the only one who looks dumb when I say, "Oh no, I do legal work, but I am certainly not a lawyer myself. Too much stress!"