Gromeer :
specs
motherboard: asus p8h61-m
ram: something with 8 gig
gpu: 750 ti oc/v1
cpu: i5 4430s
psu Chieftec GPA-400S8
i play bf4 on ultra with 60 fps no probs then suddenly my gpus power usage goes from 85% to 14% and with that my frame rate goes down to 2.5
Well the main reason is, is because your PSU is only 400 watts and will not be able to fully power it all the time. I would get a 550 watt to be on the safer side.
Also, that brand looks quite sketchy. It doesn't look very good. Try replacing it with an EVGA one or Corsair.
- Sam