if the battery checks out ok and the cables are good then you may have burned up another starter. I would take it off and have it checked out.
that's why i said to check the glow plugs and the circuit that controls them. If the glow plugs are not working correctly it will cause you to have to do long extended crank times just to get the engine to start, especially when cold starting. This can burn up a starter.
The glow plugs are there to heat up the combustion chamber so it will start better, quicker, if they are not working correctly the engine will not want to start up forcing you go continuously wind the starter till it does start. The starter will get too hot and cause the windings to fail. ie: you get a slow crank as you put it.