It sounds like you need to take it back to the mechanic who did all the work. Possibly the computer was rebuilt and is defective ( not uncommon if it is not a factory unit ) Was it a brand new computer? Have the shop retrieve the code(s) and do a proper diagnostic of the problem. There should be a 1 year warranty for all the work you had done.
I would do a sweep test of the Throttle Position Sensor on a labscope. They are notorious for a 'acting up' with out setting a code. They would also cause a 'fall on your face type of sympton'. Another thing to look at is the Ignition system. A Bad Cap and rotor and a bad igntion coil could cause theses symptons, as well as a defective distributor. I have seen Nissan Distributors fall flat on their face because the pick-ups in side get defective. They also will not set any codes, yet the car is basically undriveable. Have your mechanic check the spark output, it should be at least 40,000 - 50,000 volts. This will stress test the whole ignition system including the ignition module, cap, rotor, wires, coil and distributor. I assume that the plugs are brand new and are the factory recommended type.