Going to copy+paste another post of mine.
If you dont already have it,get
MSI Afterburner
Disclaimer: do not use numeric values shown in screenshots,tailor your clocks for your card.
Use the sliders to set your stock clocks which should be 1000 core 1250 memory(reference 290x),click apply then select save then click Profile 1.
Next drop all the sliders to the minimum value,apply then save to profile 2.
Then go into "Settings" and find the "Profiles" Tab in the top right and save your 2d and 3d clocks to the appropriate profiles,
like so.
then bypass
PowerPlay (source of your woes)
Your old 2d clocks were most likely 300 core 150 Memory,now the lowest they'll go is half the default values.
But atleast this way your full Core Clock(and potentially more!) will be used in games 100% of the time.
This should solve all of your performance problems.
After you apply those settings and give a full system restart,you should be good to go.
EDIT: Dont RMA,its just PowerPlay messing with your clocks.
EDIT #2: this should be more prevalent information,im not sure how many times ive wrote this out the last few months.