My freind had this happen on his 3rz (in a taco). I checked his fuel pressure at the fuel rail and it was too high (by only a couple psi)- which will make it run rich (i.e. bucking and loss of power, sooty exhaust, hard start....). I told him to replace his fuel pressure regulator because everything else looked good and once he did, he hasn't had that problem again. The regulator is that vacuum diaphram at the very front of the fuel rail with 2 10mm bolts holding it on - really easy to replace.
