I am sure that this problem is NOT caused by output plugins. Nullsoft's MIDI plugin simply doesn't use output plugins. I've heard that some people have problems with playing MIDI files because of lame Windows' MCI drivers. MCI (Media Control Interface) is the simplest way to play most sorts of media files (wav, mid, etc.) under Windows; Nullsoft's MIDI plugin uses it. Unfortunately, MCI (in all versions of Win9x) contains several examples of M$'s lame programming.
At first, make sure that Winamp's process priority class (preferences/options) is set to normal. Setting high priority class seems to cause very bad hang-ups while starting/stopping MCI MIDI playback.
My DirectMusic plugin is probably the only Winamp plugin that can play MIDI files through something else than MCI. You can download it from here
. And make sure that you have DirectX 6.1 or higher installed; otherwise my plugin will use MCI too.