360 timing advance problems

sounds like you're connecting to manifold vacuum instead of ported vacuum
gross oversimplification but... manifold vacuum has full vacuum at idle but loses vacuum when you step on the throttle
ported vacuum does the opposite where you have no vacuum at idle and you gain vacuum by moving the throttle
when you're connecting the vacuum advance to manifold, you are immediately gaining full vacuum advance at idle which could be adding up to 15* instantly.. then when you start driving and you want the vacuum advance to kick in, it's doing the opposite and dropping off
some people tune their cars (with big cams) to run this way but you have to know what you're doing and have the correct advanced initial timing to make it run well