distributor springs light enough ??

Well IF the spring is actually holding it back (what I was thinking) then it will continue to advance.

First thing I'd do is put one or two light springs in and find out for certain what the advance actually IS in degrees.

THEN you can determine if the distributor drive mechanism is causing spark scatter or other problems, or maybe the bushings in the distributor ain't that great.

So confirm the AMOUNT of advance, they play with your springs.