I had a problem with my 30m QCX which had some similarities to yours. I think it was a dry joint and I seem to have solved it without knowing exactly what it was, by resoldering everything in sight. One thing to check is C21 and C22. They are now '105' in version 4 - mine were buff coloured blobs. From memory, they were not easy to read but appeared to be polarised - if yours are, you might want to check they are the right way round, if you haven't done so already.