93 chevy cavalier check engine light

What is your question?

My cavaliers check engine light turns on when I start the car and stays on at lower speeds but when I stay at a constant higher speed and acceleration it turns off but as soon as I start to decelerate it turns back on. When I'm at a stop sometimes it idols down to around 500 rpms as if it wants to die. And thee is a super rough idle like rattles the car rough I know I know it's a basket case any suggestions or help? Really appreciate it.