Boy, gotta love the "occasional" problems!
My truck does the "bucking" thing you talk about, but it does it from the get-go in 2nd, or 3rd, or 4th. Pulling the choke out about a third cures it right away. Per Jon, I'm in want of a larger main jet - just haven't gotten to it yet.
Does the choke change anything when you're driving it? (sorry if I've already asked this . . . ) Doesn't really make sense with the 3rd gear only thing, but thought I'd ask anyway . . .
-Michael