It appears to me like a problem of stamina....
I had the same effect but only on Verbal.... the half way was the shift point. To overcome this, I practiced a lot, especially during non conventional time 8pm > 1am... and by doing a lot of full lenght test.
I applied also a technic : after 4 questions, I gave me a full breath with closed eyes of about 4 s.... It was a kind of reset and pretty helpful to catch back freshy mind
I suggest also to make the break as u will have them on the real thing. Drink & eat something that energy u
Good Luck !