I thought I had this one figured out until you said "if the audio plays at
all".

I'd have suggested that the video and audio were not in sync either because
of the lack of memory/number of running processes. Or that it's simply the
way some video codecs are when they are low bit rate (web quality).

The stop-starting and buffering is either because it has not downloaded that
much of the video yet - which can sometimes be helped by pausing it for a
while and letting it all download. Or it may be that it will only buffer a
little at a time and buffer more when that bit is past. Yes, this annoys me
as well.