2000 Chevy Cavalier losing power

Hi,
im running out of options, so I really hope anyone out there can help me with my problem. My cars old I know and it has a lot of miles im aware. But I just had the transmission replaced, catalytic converter replaced, spark plug, and engine/ignition wires replaced. And none of it did what I wanted originally to do, which is turn off the check engine light. My check engine light will flash and I will lose power all of the sudden, and by power I mean that I will not be able to accelerate as fast as I usually am able to. It will even go down to 500 RPM's and correct itself to 1,000 RPM's after a little while of idling. This problem has only arisen after replacing the transmission, I have had no problem with it before this and ive sunk alot of money to get all of these things replaced with no solution presenting itself, I turn to you for aid, please help me with what could be wrong?

thank you so much