GPU clock gets sometimes locked

[Mafish]

Machine : Lenovo IdeaPad z710
CPU: Intel Core i7-4700MQ
RAM: 6GB DDR3 @1600Mhz
GPU : Nvidia GT745m

Hello everyone,
I've got a problem with my GPU.
While playing BF4, my FPS were going crazy (totally random), sometimes i could get min fps of ~50 at 1080p, and the other day I hardly go to 30fps at 1366x768.
With evga precision , i've found out that gpu clock speed sometimes stays low, at 405Mhz , and sometimes jumps to 810Mhz. 
First thought : temperature , but it stayed at the same level on both 405 and 810 Mhz.
Second though : power , but the powerbrick is rated for 135w and it drains only about 100W from the wall.

I've already seen a lot of posts, ppl having same problem but none of their solutions worked for me (i've done all basic stuff like restarting pc, clean install of drivers, reinstalling game, turning off power saving etc.)

Similar things happen in other games (Witcher 2, WarThunder) but in less demanding titles (LoL, CS:GO), clock speed sits at 810Mhz, and GPU stays at nice and comfy 60-65 C.

I've got no clue whats wrong, any ideas ?