hi! something weared happed in here, my project

I just add some force when the play press right and left arrow key to gathering with force mode so the player will fly like jetpack connected. nothing dought on this, but my problem is player behave two types with the same input please see the image from (1.gif and 2.gif)

here is my problem occur when I play with a minimized window the player will behave one type and I pause playing and maximize the window and when I resume it then the player behave another type, but I don't change anything to the fore or gravity anything, but player behaves differently with minimized and maximized window what is going wrong here. plz someone help me. I can't able to find the correct force to make the player fly better.

[2]: /storage/temp/164094-1.gif