Is this a stock engine? Any mods like the Lexus AFM? Do you have an adjustable fuel pressure regulator or S-AFC so you can adjust and tune the A/F ratio?
You can also check this by using the diagnostics port and hooking a multimeter to VF and E1. The voltage from 0 to 5 volts also is an indicator of rich or lean.
0.0 = rich
2.5 = just right (stochimetric)
3.5 = lean side
5.0 = lean
I would do this check also to make sure your gauge is working correctly.
My guess is the fuel pressure is too low <-- shot in the dark. :burnout: