Tahoe running rough

I have a 98 Tahoe with the 5.7L that is running rough. It has a rough idle, and surges some. It has been throwing code P306 and once in awhile P300. I am home on R&R so I dont have much time to figure it out. The dealership did a tuneup on it a few months ago, new plugs, Distributor cap and such. Any ideas would be a great help.

Also have another question. How much vacume should there be in the engine itself. Im not sure if its related to the running rough or not. But there seems to be alot of vacume in the crankcase. If its running and I remove the oil filler cap or one of the lines that run into the valve covers it will idle way down and die. If you put your hand on one of those line or the filler it will suck it on and stay running. Used to have a 98 Silverado and I dont remember it doing that. Thanks in advance.

If you pull the PCV valve and the breather tube and block both of those holes in the valve cover there should be no vacuum in the engine with the vehicle running. If there is that there is an internal vacuum leak possibly around the #6 cylinder since that cylinder has been specifically identified as missfiring.

If there is no internal vacuum that you can still concentrate around the #6 cylinder. Checking for spark/compression, fuel.

So plug those two lines and see if there is vacume at the filler cap?

You can plug them with your thumbs/fingers. If you feel vacuum in those holes with your finger(s) than you know your on the right track.