Possibly the Vulcan physiology makes it easier for them to suppress emotion (different Neurochemistry, better control of it?) and being half Vulcan makes that biological control harder?
Possibly, but the violent Vulcan backstory and existence of Romulans seems to speak against it.
I always thought that when some Vulcan called out Spock's heritage, they were being rhetorical. If you were the son of an American woman and a Tibetan monk, and living in a Tibetan monastery, every time you showed any signs of impulsiveness they would say "oh, it must be the American in you." If you lived in America, anytime you acted serenely they would say, "Well, it is because your father was a monk!"
I think that Spock grew up an outsider, someone who was reminded constantly of his heritage, and so when he went off to live with humans he found that he could identify with humans even though he sometimes wished that he didn't. I mean, it is obvious that he did, why else would he serve with them for so long? It isn't really anything to do with his blood-- that's just a convenient metaphor that even he himself uses-- it's his perceived heritage. In reality, any Vulcan that was brought up by humans would behave as a human would, or at least they would have the same difficulty controlling their emotions that we all do (he/she might actually have a bit more of a tendency to be violent).
On Sarek: I have always thought that when Sarek said "So Human" he was talking about himself and just projecting it on his son. After all, he was impulsive enough to want a human wife and impulsive enough to say it. I think that Sarek was going through hell when Spock was born; he was an outcast himself, having given up his love with a vulcan princess to marry a human woman and have a child with her. What must everyone have thought! He was a shaker and a mover, and he clearly was quite emotional sometimes (Why have you left Spock on Genesis!?). Why was it that Sarak was ambassador to the federation? Maybe it was because they thought it was the only job he could do. "If you love humans so much, why don't you marry them? Oh, you did, well, why don't you go live with them too."
The fact that Sybok, a full-blooded vulcan abandoned the way of Surak and embraced his emotional side shows that the household Spock and Sybok grew up in was one of contridiction, where they were forced to embrace pure logic at the same time they were being taught, through the actions of their father, to be emotional and impulsive.
I don't think it has much to do with genetics at all. Sure, instincts are instincts, but I think that the house of Sarek is one of emotion and sensuality, and that was passed to the next generation. Spock eventually decided that it was in the benifit of his people for them to be reintroduced to their emotional side via reunification with the romulans. To him, this contridiction, this walking of the line between pure rationality and pure impulsivity, was enlightenment, and as much as he tried to deny it, and tried to supress it, that really is how he lived.