i5 2.8, 4gb, 5750 upgraded to i7, 16gb, 6970 - stuttering graphics?

I have upgraded my mid 2010 imac i5 using a bunch of guides on here from:

• cpu: i5 2.8ghz
• ram: 2x2gb
• gpu: radeon 5750 1gb

to:

• cpu: i7 2.93ghz
• ram: 4x4gb
• gpu: radeon 6970 2gb

(+second SSD installed under optical drive)

the system was running fine with only the ram, SSD and cpu upgrade, but since i replaced the gpu, i get stuttering performance in games, i.e. the frame rate drops to about 5fps for 10 secs and then comes back to normal for 10 secs before dropping again.

this happens in all my games and both in os x 10.9 and bootcamped windows 7. Temps are around 80°C, which is similar to my old gpu.

when in a game's 2d menu that has a 3d background, the cursor will stay responsive while the background stutters.

my thoughts so far:

• faulty gpu?
• wrong drivers? (amd auto detect didnt get the name of the card but said drivers are up to date)
• psu not putting out enough power? (all the upgrades require more watts)

any comments? i really dont want to put the old card back in...

[edit] Using ati overdrive, i found that the card will fall into low performance mode which means 100mhz gpu clock and 149mhz memory clock. how do i stop that?!

[edit2] I was able to force the card to stay at normal clocks in bootcamp, but GPU-Z read nearly 100°C on one of the sensors.

I have removed the card and reapplied thermal paste.

Also I have moved the SSD away from under the optical drive because it was blocking the air flow to the gpu heat sink. It is now sitting next to the HDD under the gpu'd heat pipes.

[edit3] I'm using Macs Fan Control to make my fans spin up earlier because the card doesnt like temperatures as high as apple's default settings do.

Max temp is 78° while under heavy load, thats good enough. fans at max speed obviously...