nopulse,
i have similar problem: after playing a while, suddenly head moves upwards (seeing only blue skies and clouds) and steering against it is like working against a decalibrated joystick: when centered, crosshair goes back up towards heaven.
it stops when i change to weapon-sights (optics).
but back to normal view it's just again blue sky, clouds and enemies killing me ...
mouse control in menues is normal but returning to game the problem is still there until exiting and restarting opfp.
next time i'll try alt-tabing to windows and back; i hope it helps.
eetee