Rough idle, engine miss at medium throttle

What would be the best way to check for leaks around the intake?
I'm not really sure if the PCV valve is working properly.
It was just a thought.

Engine: 4.3L
Auto Trans, 2WD, mIleage is about 215,000

This rig has a Goddwrench motor in it, not sure of replacement mileage, bought it used. It was at a very low idle and stalling a lot. GM tech scanned and had no codes. Timing is OK, EGR is new, IAC was replaced last night, it still doesnt run right. Now it seems as if a vaccum leak is the problem, but no ideas. When the heater or vent is on the idle deteriorates, but turning AC on bumps idle up to an acceptible level. My next guess. MAP sensor? Any help would be appreciated. Thank You in advance.

Thank you for the info. I am about to lose my sanity over this thing. I will check on those tomorrow and see if that helps. If not off to the looney bin.

So today I took the throttle body off and the EGR passage was plugged. The gasket was toast and some less than brilliant light buld drilled out the idle plug and messed with it. Soooo.$50.00 and about 6hrs later I think I may have solved the problem. Also, new fuel filter. And then the kicker driving it around the block and asll the "do it yourself" spark plug wired that came with it fell apart. So. I think I fixed it. Time will tell.