I’ve had the same stuttering on multiple file formats, not just FLAC - more than enough upstream bandwidth at home to support FLAC, let alone 192kbps MP3.
I have also found a workaround - if you are running Plexamp on a network that has a (relatively) fixed IP, you can get your Plex server to assume that it’s a LAN client by adding an entry into the “LAN Networks” field under Network (with advanced setting shown).
e.g. if your external IP were
a.b.c.d/32 into the LAN networks field tells your server that it’s actually on your local network, and the regular bandwidth rules no longer apply. This seems to eliminate the problem - I spent a good hour listening to various FLAC tracks from different albums, with pausing, resuming, skipping between; no stuttering.
Caveats: this only applies when you control the server; when the client where Plexamp is running is on either a static external IP, or a relatively consistent range; and when the files you are listening to are not higher bitrate than your server’s upstream connection.