Subs are shuttering

Yesterday I  uoloaded a .mkv file from my pc to my live hub, witch I bought a week ago (to replace my Iomega screemplaydisaster).

When I wanted to see the file the subs stuttered as if the lifehub was working on the background.

Even the playbackoptions were slow (took about 2 sec. before FF or Stop reacted)

Another .mkv file uploaded a week ago had the same problem ( subs were sync but some lines where not shown ore shown for a fraction of a second).

After a restart all went fine.

Used  mkvmerge Gui to muxe in the subs.

Wait… So you have to restart and then the subs are working fine? o.o