P0507,P0306,P0300 codes 97 Suburban 5.7

I just bought a 97 GMC 1/2 ton 2 wheel drive suburban with 165,000 miles. I drove it 300 miles home with no problems but the SES light was on and before I bought it I checked the codes and got P0507,P0300,P0306 codes. I replaced plugs and rotor and cap. no difference. I am not losing coolant but it does use a little oil. The idle seems to vary from 1000 rpms to 1700 rpms but it goes down the road fine. What would be the next step? any help and ideas are appreciated.
Thanks,
ccnova

P0507 means the Idle control valve has to move out of range in order to reach acceptable idle speed. The P0300 is a random misfire and 306 is cyl. #6 misfire. All these codes together makes me think there is a vacuum leak somewhere on the engine. It could be the intake manifold gaskets or any vacuum hose on the engine with a break in it. IT's not uncommon for the throttle(butterfly) valve to leak at the shaft. To check you can spray throttle body cleaner (COLD engine) around the throttle body's exterior with engine running and around your intake gasket area. If the engine stumbles and the fuel trim readings immediately drop into a negative double digit range= then you've found the leak.

I failed to mention in my first post that a friend had told me to plug the holes on the valve covers and check for vaccum on the dipstick tube. I did this and there was vaccum present. He told me this would indicate an intake leak. Is this a reliable test or not.
Thanks again,
ccnova

If you plugged all the valve cover ports AND the oil fill cap if it's vented and you have a vacuum at the dipstick tube, then yes, your intake gasket is apparently leaking into the crankcase. It's not visually verifiable, though, but it sounds like your friend is right! That would also explain why you are using oil.