Constant frame rate drop

[Cirrus698]

I have a constant rhythmical frame rate drop when playing the game.
Frame rate goes from 70fps down to 25fps then back up to 70fps on a constant rate.

I have graphic setting to medium, I tried with anti-aliasing off, v-synch off, but no change.

I beleive I should be able to run the game without any problems
CPU:AMD Phenom II X6 1090T Black 3.2 GHz
GPU: AMD Radeon 6970 2GB DDR5
RAM: 8gb DDR31600mhz
For an instance I can run BF3 with all graphics setting at max with 60fps

Please help.