3/5NL Live: River Trips Faces Big Overbet

[beset7]

Last one and I'll respond to 5 to 10 hands posted by others.

3/5 NL 9handed. Stacks are about 800 on this one.

SB is loose agressive. In PT terms I'd say he's like a 50/20/3. He's been check/raising me on the flop a lot. I've never caught him bluffing but that doesn't mean much as we've only played about 20-30hours together.

Its folded to me in the CO with KQo I raise to 20 (my standard pfr). Only the SB Calls. Flop comes 9 6 4 rainbow (suits unimportant). SB Checks, I check behind. Turn is a Q. He checks, I bet the pot, he calls.

River is another Q, he bets 400, I look at the little pot out in the middle and the four stacks of red he just pushed over the line and I...

Need to know anything else about SB? I'm really interested to know what sort of things run through your mind in spots like this. How do you reason towards your decision in this most annoying of spots?