I have a 1974 Soverel 30 Mk III, which is an absolutely great boat. She performs well & fast on every point of sail. Two things to check are:
1)the swingkeel cable/shackle and pivot pin --I replaced the quite frayed stainless cable and shackle on our boat this year (the cable was the original one, though it was evident at some point the shackle had been replaced, but not very well.
2) the bottom pivot plate for the rudder, to make sure it is tight and solid. Even though it is not weight bearing (at least on the Mk III model), is is subjected to quite a bit of lateral stresses.