thats true,
I got it working when i dont stop the timer in its overflow ISR.
It seems that it is not possible to stop it there, because
when i set the Timers count or not count bit to zero in the ISR. the timer calls the overflow before it actually has overflown...
i dont know how to solve this problem. but i recognized, that i dont have to stop the timer because it wont go further if I dont reset TCNT3.
so resetting it when I want it to gives it a next round.
-Flo