Sounds like your timing is changing when you put the car in gear, either from your mechanical advance starting too early (shouldn't start until about 1500 rpm), or because the vacuum advance is hooked to a manifold source instead of a ported source. You didn't say whether you set the idle speed with the vacuum advance connected or disconnected, and whether you have it hooked to a ported vacuum source or manifold vacuum source.
Unplug the vacuum advance, set the idle speed and check the timing in neutral, then in gear (warmed up of course - set the choke and fast idle after getting this figured out). If there is no change in timing and the car now has a reasonable idle, you're good on the mechanical advance. If not, put heavier springs in until the timing doesn't change.
Next, plug in the vacuum advance, If you get a noticable increase in idle speed, you probably have it hooked to manifold vacuum. Find the ported vacuum tube on the carb and try that.