Interesting. If richening the mixture increased your idle 250-300 RPM, then I bet the idle mixture is still too lean. As you are probably aware, getting the curb idle and curb mixture settings corect is a dance between idle mixture screw opeining and curb idle speed screw setting.
20-22 inches of vacuum is great for idle. However, the fluctuating vacuum at idle is worrying. The vacuum, especially at idle, should be solid and steady. Where are you tapping into to get your vacuum reading? Are you drawing directly off the vacum tree on the #6 intake running? That signal can cause a vacuum gauge to jump at idle since it will ready the pulses in the #6 runner. A better place to tap for an accurate reaing is the hose that operates the vacuum choke pulloff. Install a T fitting in the middle of the hose.
Most of your issues sound like a lean running condition. I bet cleaning the pasages out and richening the idle mix got you much closer to solving the problem. Spend some time adjusting the idle mix and curb idle speed screw setting to achieve maximum vacuum at about 700-750 RPM in park.
