I'm not from US of A, i have to deal with 3rd world countrie customer service here.. And the battery cappacity isn't an issue, it's the ___ random drain that happenes from time to time which now totally ruined WP8 for me..
And li/ion battery don't need those 3 time charge/discharge.
Boris, you are absolutely wrong, wrong, wrong about lithions not needing the 3X charge discharge. I have been an amateur electronics person for over half a century. I have built camcorder battery packs way back, custom camera packs, and more. I've been through the original Gates branded new technology of spiral wound gel cells, nicads, NMH, and lith-ions. I have never seen an outta the box lithion that didn't get increased capacity from the 3X charge/discharge. Never. Sometimes the increase is minimal but noticable, and sometimes, like my new 2500mah battery recently, went from 12 hours of run time to 4 days.
I have had one incident of the sudden discharge rate you're experiencing. While you tend to think of the battery as a passive partner in your phone, it is not. It has a tiny chip in it reporting things back to the phone's power center. That's why there are three, and sometimes four, connectors on the batteries. This is the old Sony Info-Lithium system. Your phone could be consuming a lot of power suddenly, or the chip could be reporting falsely. In other words, there's still a lot of "juice", but the phone has been told there isn't.
I don't recall if you've addressed it before, but when this power surge happens, does the battery get warm? That would tend to indicate the problem is in the phone. Nor do I recall if you've done a hard reset. Sorry if the answer is "Yes."
I've given several real world tips to you, but you seem to rather keep complaining. What have you got to lose by doing the 3X system other than monitoring time? It's free, for crying out loud. I suggested you get another battery, preferably with a stand alone charger. I don't know about customs duty where you live, but if it is zero like here in the states, you can get TWO batteries and a charger for $10 or so. And you know what? If you won't send your phone in for service, then maybe your best option is keeping charging batteries and keep one in your pocket. I've done that in the past with two phones in particular.
As the gentlemen above suggested, you need to log these things if you ever expect to get help from anyone. Nokia or here.
A lot of people have tried to help you, Boris. It's time you figured out your course of action.