Ok so I've tried clearing cache, no help.
Tried these two avi export options:
MPEG4 (Native)
Compression settings - default
Sound: MP3 (Native), 44100 Hz, 320 kbs
and
H264 (Native), Full quality
Sound: MP3 (Native), 44100 Hz, 128 kbs
also no help. Video always lags at the same spots, too.