Why does my engine stall?

Today the following thing happened 6 or 7 times:
When I slowed down to a full stop the engine stalled. It made about 3000 revs, I breaked and applied the clutch, the dial of the rev counter went from 3000 to about 600 and the engine went dead.
A couple of times I released the clutch and the engine started again, without turning the key.
I have recently changed the airfilter. But that was a week ago and the problem first occured today.
Apart from this the engine works perfectly.
Do you have an idea of what might be wrong?