Ron, I went back to 1571 (Windows XP). The rebuffering seems to be gone, but the FLAC–>WAV transcode to a Soundbridge for newer FLAC files (1.2.0 and 1.2.1 – I tried both) is not working, suggesting as you said that the problem is with the ffmpeg software.
Poking around on the Web, I saw that Josh Coulson recently submitted a patch to update ffmpeg for some FLAC changes. In playing around with different “solutions” I learned (having tried) that we users can’t just overwrite your versions of the ffmpeg-related dlls with the newer ones and have a go. (All transcoding stops.)
I also experimented with overwriting your libFLAC with the newest one. In this case, Firefly still transcodes the older FLAC formats, but not the newer.