2002 Toyota Corolla engine check light on issues

Hello, Thanks in advance for the advice.
For the car, changed the catalytic converter (CC) and the 2 front/rear O2 sensors in Aug 2007 after the code came up on the diagnostics. After changing the CC, the car appeared to stop sometimes even with my foot on the accelerator especially when I am ready to set off from the stop light. Strangely, the engine check light came on for a few days and then disappeared. The fuel injector was changed by the mechanic to address the stop/start problem mentioned above. It appeared to help, but I seemed to sense the problem sometimes. About a week back, the engine check lights came back again and this time, the codes indicated slow heated O2 sensor (code P0133). It was replaced and as soon as I went up high speed, the engine check lights came back ON. The mechanic re-diagonized to be a problem with the car's computer and that ONLY a dealer can "re-flash" the computer and figure out what the problem is. What seems to be the problem and how do I deal with it? Should I go to the dealer which I do not want to!