[Slobodan]

I had this same problem on a customer's car. It might not be what your problem is but its something to consider.
The alternator would start working harder and harder because of a build up of resistance that was happening at the B+ hook up on the back of the alternator. This was happening because of a very old and corroded ring terminal connector. Solution: Cut back about an inch or so on the cable and crimped a Copper ring terminal connector on instead of the aluminum one. Problem solved.