Issues about fps on my laptop

Hi.

I need some help.

I've read a lot of threads about Nvidia Optimus and similar threads, but my problem is a little different from what other said.

I've an Asus N550 with Nvidia Optimus , so I have an Nvidia 750M 4GB DDR3 and Intel GMA 4600.

My BeamNG.Drive, detects Nvidia, and it's using the Nvidia chipset. I've done some tests, and yes, the game is running on Nvidia.
What I have? Game run's like 5 seconds fine, and then another 5 senconds at maybe 5fps... then again, 5 seconds very good, and again 5 seconds at low fps. In every scenario, even with setting to lowest, or to maximum detail, it's exacly the same problem.
But, if I disconnect the AC charger, it runs at 20~30fps constant, without drops. If I plug again the AC charger, it start again the 5 secs very good, 5 secs very bad...


Another issue that happen is that when I start the game, it changes my brightness , and when I close the game, change again my brightness, like if it was changing the energy plan of my laptop. This is normal?

Could you please help me?

Note: This happens since I bought the game, half year ago, so it's not a problem on a particular version, or a particular nvidia driver version too.
On GPUZ what can i see is that the gpo are on a load of 95% when the game runs well on that ''5sec'', and when he runs very very slow, the gpu usage goes to 5%, so probably it's not an overload of the gpu, since even in Lowest setting, it happens.
The CPU never passes 20%, and about RAM, well, i've 16GB so that's not a problem too.
Another important detail, i've an Samsung SSD, so the disc, should not be the problem too.
All drivers are up to date.
Maybe he's switching my power plan when starting? (But yes, with the game running, power plan says perfomance optimization)

 

Maybe because charging causes more heat and your laptop starts protecting from overheating.