I bought a rebuild manual by George Reid C4 and C6, mainly because I want to keep my car as original as possible when it goes through the testing grounds and once registered it's out with the auto an in with my 5 speed. He mentions in the trouble shooting section with early or late shifting between 2 to 3, that it can be the vacuum modulator or rod malfunction, also valve body malfunction or governor malfunction or loose tubes on the governor distributor. Makes no sense to me but maybe to you. I have a damaged parking pawl on my C4 and that why I bought the book.