Slightly Low-High rpms and chaking. Please help

[ALxSantizo]

Hi I am having a problem with my z24 1998 automatic chevy cavalier. The problem has been like that for about a month.When I start the ca, it has low rpms(500-650) and moves back and forth between 500-650 quickly , as soon as I tap the accelerator it goes directly to 1500 rpms and stays accelerated for a long time (3min) until I accelerate it. It also chakes alot. When I turn off the car it make a puff sound ( it sounds to me like not all cylinders stop at same time, but dont trust me on any diagnosis I make) .The problem is only noticeable when on idle or first gear. I replaced the sparkplugs,the plugs that hold them, air filter, and ECT(engine collant temperature sensor). I took it to a diagnostics test with a friend of mine that works at a mechanic shop, the diagnostics test says something about the ECT , thats why I changed it , it helped a little ( it stopped accelerating by itself) but the rpms are equally low and it still chakes altough it got better. Also the check engine light turned off when i replaced the ECT. But the problem is still there.. .

I do not know what else to do. I can't keep replacing random sensors and parts. And I do not have enough money to just give it to a mechanic and risking him not fixing it completely. Please Help. Also, the oxygen sensor attached to the manifold was disconnected when I bought the car and It never gave me any problems , it is still discconected, I would like to know if the O2 sensor can cause that kind of problems or what other sensors or things can I check for. Plese Help. Thank you for any help in advance, i greatly appreciate it.