Wouldn't an air leak cause it to run rich rather than lean?
I had something similar happen on a previous Supra and it turned out to be a dud Bosch fuel pump, which turned out to be an easy fix. Therefore, my immediate hunch would be to check the fuelling system; fuel pump, fuel lines, fuel filter and injectors for issues then move on to the electrical side of the fuelling system. I.e. check the wires to the fuel pump for unexpected voltage drops and check the wiring loom going to the ECU for damage or kinks in the wires, as I have previously experienced issues arising from the same, albeit on aftermarket ECUs and harnesses but still worth a check...