2000 Volkswagen Eurovan Starter

Engine Mechanical problem
2000 Volkswagen Eurovan 6 cyl Front Wheel Drive Automatic

I started the engine. Nothing unusual happened, however within about 30 seconds the starter engaged by itself. I turned off the engine, raised the hood and looked for signs of trouble. Did not see anything, but there was a faint smell of electric insulation. Tried to started again. The engine started immediatelly, the starter disengaged correctly only to engage by itself in a minut or so. Turned off the engine. Than the starter engaged on its own. I disconnected the battery and I don't have a clue where to start. Would you please comment? Thank you Vlastimil.