I used to have the same problem, ie: good or even too fast idle, then when I'd put it in Drive...HUGE rpm drop, sometimes to the point of stalling. The car doesn't do that anymore, and I can't be sure which adjustment did it, but what's mentioned in the above posts is the same tack I took....minimize/eliminate vacuum leaks, use a vacuum gauge connected to manifold vacuum to set your idle mix screws for maximum vacuum at idle. Set your timing for 15-20 degrees btdc (mine was still gaining rpm all the way up to 40 btdc...so much for timing by ear) with the vac adv can disconnected and the hose plugged. You can reconnect it to see what your total advance is, but that doesn't really pertain to the subject of odd-ball idle behavior unless your distributor is connected to a manifold vacuum source. In that case you would be getting 20 or so degrees of advance at idle, and no more when the rpm goes up. Also, as said before, make sure your choke is opening all the way, and of course that your vac lines are connected to thier correct vacuum source (vac adv dist uses ported vacuum, aka before the throttle blades. A vacuum gauge for a reading should be after the butterflies). Clogged pcv, bad brake booster, dirty air filter, float level, crud in the needle/seat (Edelbrock equivalent), etc. Lots of settings and adjustments you can tinker w before you really have to take anything apart.
