Chaps,
My P3S has a very werid issue. When i dont fly for a day or so and i take off, the drone starts to shake / wooble. The shake is coming from left back motor (I also have a stress crack there), but after like 2 mins it goes completly out. No shake, completly stable. Anybody has an idea why is this happending? I fly every day and it allways happends.
What can i do to stop it?

Have you tried repairing the stress crack?
You may have a faulty motor have you tried running the motors with props off does it still shake/wobble?
Can you upload a short video showing this shake/wobble

I agree with Hellsgate.  Must be the motor bearings, right?  If it was a prop, it would do it the entire flight.  If it was the crack, it would do it the entire flight.  The bearings and grease heat up and the vibration evens out.  

The crack was likely caused by the vibrating motor, it seems to me.