the answer is no, even nokia care won't downgrade the firmware for you.
mine has big problem since i updated to v20 via NSU...sometimes it is unusable at all, sometime can use for a few hours before it's reseting/restarting and dies on its own.
i have tried various methods to revive my phone, countless hard reset, reflashing with various firmwares..still the same problem occured...
i have damaged the ori memory card in the process ..it is corrupted, cannot be re-format using PC and the phone itself

i've tried to change the battery but apprently there's no AP battery yet for  nokia 5800, ori battery cost like RM200
but  i found cheap BL-5j battery for tube on ebay, bought just now.. only USD6.50(free shipping).
i keep my fingers crossed it;s the battery that gives problem to the phone