2001 Toyota Avalon

Engine Light Came on, noticed black smoke. Mechanic replaced the MAF sensor. Two days later Engine Light Came on again same error P0101. Took to a Toyota place and they tighted air flow meter terminals and clear code. Light Came on again. Code P0420. Took back to mechanic and they cleared the code. Car was running okay. Two days later the car had performance issues again. RPM goes up but doesn't want to move and engine stalls. This time there was no black smoke. Back to mechanic. They removed intake mainifold and open wiring harness up. Traced wires from air flow meter to ecu. Checked and tighted all ecu pins dealing with air flog meter. They resecured and closed wiring harness back and reinstalled intake manifold. Performance issue seems to be resolved. Cleared CEL code. Two days later the light came on again. Any suggestions. The mechanic is stumped. He said it may be the ECU but they rarely go up so he is not a 100% sure if that is the problem. Any suggestions.