Definately sounds like a vacuum leak. When pulling the intake (I don't to clean the egr tube) you should replace both breather hoses 104-094-35-82, and 36-82 as well as the intake runner seals, they are not expensive. I had a#5 intake seal drop onto the starter during manifold installation once. Didn't notice it until the idle was goofy. Check carefully under the intake and between the block and starter to make sure one didn't drop. Another sneaky vacuum leak is where the purge line connects to the throttle actuator. Sometimes the line has tension on it and fiddling with things lets it pull loose.