My check engine light came,the car was bucking while driving, car wouldn't start on 2005 Nissan Altima

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at different times after driving and parking it, it would take 2-3 times to get it started sometimes after 5 mins before it would start again, also would stall out after starting it and would stall out while idling at traffic lights or when making some turns and would take 2-3 times to start again. The mechanic told it was the catalytic converter, it will cost $969.00 to fix. Needless to say I went ahead and had it fixed, the check engine was off for 1 day, it has come back on and it still same symptoms as I stated above. What is problem? I believe I have been ripped off, Not sure what I should do, any advice?
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