If it is slipping as you describe, it is either the balancer is broken OR the timing chain is jumping. Either is a mystery since a jumping timing chain will quickly get you a cam that is seriously out. A slipping balancer will get you a problem because, if you line up the timing to the correct amount, the timing will be off (because the mark slipped).

Check list: vacuum advance disconnected...right?
Does the timing ever "slip" the other way? It seems like you have described it slipping at least several times.....Do you notice if it is ALWAYS advanced too far?