Thanks for your response. I'm sorry to hear that you have similar problem.
I think that this part causes our problem (but of course we have to be sure of it):
I have to take look later (read: get inside camera) and if this is a problem, I'll need to buy this part and replace it.
I'm afraid that Sigma doesn't provide repair for this model any more. And also cost of repair can excess cost of camera. But this is of course my speculation.
Anyway, I'll keep all folks posted what's cooking.