Received some more 3870 chips..
They acted exactly the same as the first chip under sub zero temperatures so I became a bit suspicious.

It seems that it is not a cold bug causing the trouble but the new AGESA version.
Most likely the new version synchronizes some other frequency with the SCLK.
Obviously this other frequency (LCLK most likely) cannot tolerate frequencies high enough to reach the SCLK limits.

This is most likely the case since I´ve heard reports about poor SCLK overclockability on Asrock and MSI boards.
Need to get some more info.

So even with A6-3670 and A8-3870 APUs please use the old bios versions (eg. 0802TS for F1A75-M PRO).