Agree Whitey! I had this happen on a Ford Granada; squealing belt and it was the A/C compressor seizing up. I put a smaller belt on used for the car without Air and lived happily ever after without air. Sold the car later for $700.
Since you stated that the battery voltage drops down to around 12.3 volts when the slipping happens I have to wonder if the something is loading the alternator down to the point that it canāt keep up with the current demand, instead of the alternator not working during that time period. That would put a lot of stress on the alternator and the belt. It would also cause a huge current draw on the charging system and I am not sure what could cause such a high draw of current. The only thing I can think of is a faulty battery cell possibly. If there is a high current draw occurring and causing this issue, then I would think that the main output lead of the alternator would get fairly warm to the touch.
I have a friend that had a similar problem, it turned out there was a small power steering fluid leak, just enough drips on the belt to cause a problem.
Youāve got a couple good ideas. Iāll just throw in that my Olds had three belts and I donāt remember anymore what ran what but my compressor seized. I never fixed it but either the belt burned off or I cut the belt off. At any rate there was no way, no matter how tight I made the belts, that I could get the squeal out of the alternator belt first thing in the morning. It would squeal for a block and then heat up enough to grab better and be quiet after that. Yeah everyone knew when I was leaving for work. Sold the car for $200. I suppose at that point with 400,000 miles could have been bad pulleys too, or the lack of a belt pulled it out of alignment, I dunno.
@db4690 Hi,
As I mentioned, both belts in car are v belts. Car does not have an automatic tensioner. Manual has a diagram with puling force in kg/lbs and deflection in milimeters/inches. It takes luggage scales, ruler and some dexterity too measure right tension. 
Hi, There is no possibility for this to happen in this car. Power steering pump is in engine internals, piping is in other side of the engine. Its longitudinal mounted engine.
@Cougar
Hi, Thanks for your answer.
This morning, before starting a car I plugged in a smart battery charger and raised V to 13,6. I started a car, and there was no slippage, It could be coincidence, because belt is currently tightened as a bow string (The looks of neighbors at 6 a.m. is too much to bear, even more than damaged alternator bearings :)) I think your theory might work out, and as it is not expensive to check, I will borrow a good battery and release belt tension in weekend and let you all guys know.
Yeah I know what you mean. When Iād leave for work, my neighbors would hear the squeal and say there goes Bing. I guess when you are young, you can take a little humiliation.