Idle/ Maybe o2 sensor
 

So about 2 weeks ago I replaced the rocker cover gasket. Once I put it back on I started the car. The car fired up and slowly warmed up. The RPMs reached idle speed the car started to idle really badly. The car sputters from the exhaust and when you press the accelerator the car misses a few times then catches up and revs but once you let off the gas the cars idle speed drops to low and the car stalls. I had a Z guy come over and check it out put he only said it might be the O2 sensor but when I take a look at the connector it looks brand new. Please help.