99 cavalier z24 " missing" help please
 

Hi everyone, new here but the last 3 days I have been searching and searching so now its time to ask for help.
My wife has a 99 cavalier, and 4 days ago it start missing really bad.
At idle it jumps from 1k to 800 over and over, if you give it slight gas it goes away.
First I thought it was the transmisson had it checked... dude said it was the motor for sure.
This motor is 4 years old from a previous owner who had a rebuilt engine installed. Its been flawless up until now.
2 years ago she had the timing chain done, and they did the upper coil piece because they broke it.
Last summer we did spark plugs and air filter just regular maintenance.

So heres the issue, I changed the fuel filter, the iac I cleaned it ( it wasn't very dirty ) cleaned the tps. Im totally stumped.

The car if you baby it up to 25, 35 50 even it will cruise right along, won't miss at all. but the time you give it gas it either? bogs out or " slips "
dunno what it is... really weird.
My only other thought is maybe the timings off a tooth but if that was true wouldn't it spit and spudder at speeds also?

Thanks in advance everyone.
steven