ANTO,
Very nice looking swap. I just got my 1UZFE running in a 1983 Celica. I know how much work it is.
You engine stumble at 2500 to 3000 rpm is probably a lean condition or it could be weak ignition. Can you measure the voltage from the front oxygen sensors while driving and see what they read? Under acceleration, they should stay above 0.8 volts. Under steady driving, they should switch up and back from under 0.2 volts to over 0.8 volts several times per minute. If the voltage does not stay over 0.8 while accelerating hard, then you are not getting enough fuel into the engine. 2500 to 3000 rpm is very close to the maximum torque area of the engine, this is where it is pulling in the most air per rotation. A small vacuum leak in the intake between the air flow meter and engine could cause this type of a problem. If you are going lean, and can't find any leaks, the next thing to check would be your fuel pressure. If your pressure is dropping under load, you may not havve a powerful enough fuel pump, or you may have a restriction in the fuel feed line. If all of the fuel system and intake check out okay, then you may have an ignition problem. The mixture in the cylinders is the most difficult to ignite when the pressure is the highest. This is also at the highest torque rpm range. Check the resistance of your high tension leads from end to end. I will have to check the book, but there is a range of ohms per foot of length that is acceptable. Also check the distributer caps and rotors for burning, cracks, or carbon traces. Miss fire due to weak spark can also give false O2 sensor readings. The fuel and air is not burned, so the sensor just reads the air as a lean condition. This can be very hard to track down. Good luck getting it running right.
Gary M.