[F1 2012] FPS/GPU usage drops

[SmOgER]

Q6600 @ 3Ghz
HD 7850 1GB
4GB DDR3
750W PSU
Win8 RTM

1680x1050
graphics maxed out obviously (vsync on).

OK so for no appearent reason when driving on the START/FINISH straight, GPU usage drops to lows 60s%, FPS drops from 60 to ~40.
CPU usage never seen higher than ~75% there, so no room for ANY bottleneck. This is so annoying that game becomes practically unplayable...
Tried lots of drivers, results were the same.
What could be the possible reason for this nonsense? I'am confused
btw, when FPS drop occurs CPU usage rises from 60s to 70s. Core parking disabled in windows.



PS. The highest GPU load seen in this game was about 80% (when getting 60FPS in other part of the track), so it is not 'usage wall'.

PPS. Temps no higher than 50C

 

[SmOgER]

What is more, I noticed that setting vehicle reflections from ultra to medium decreases FPS drop by a great amount, but it's not really that kind of game where with HD7850 you should look for compromises, especially at 1680x1050 and laughable GPU usage, which becomes ~40-55% with reflections on medium

Any ideas?