Not sure if I've missed an option in the PMDG options/settings somewhere, but if the autothrottle is armed, shouldn't the aircraft automatically add power when the speed hits the minimum maneuvering speed (top of the yellow line on the speed tape). Maybe it's an airline specific option?

Do you have this problem after take off while flying manually and following the f/d??if so do you have cutback selected?and do you have vnav engaged?or is it happening with all the automation selected and during other flight phase?

No. If an impending violation of the buffet speed is detected then a reversion occurs if the pitch mode is VS, to level change, or VNAV PTH to VNAV SPD (if the AT is armed then thrust is adjusted accordingly). EFSM activation (elevator feel shift module), auto-slat and automatic nose down trim are further actions occuring mostly between the red and the yellow limit (stall/buffet) or even deep stall. All protection mechanisms are modelled.