Crankcase issue

[fast400]

With the car sitting at idle I pulled the oil fill cap. As soon as I did the idle dropped and the car ran rougher. As soon as I put the fill cap back on, it ran smooth again.
Is there a problem with the pcv system(fairly new valve)? It shouldn't act like this...should it??

 

[boostingbuick]

Seems like a big change for removing the oil cap. The air that is sucked through the pcv is not metered. It will act like a vacume leak. I would inspect the breathers to make sure that they are clean.

 

[fast400]

No breathers...stock setup.