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When holding down the Shift key to look ahead, the mouse sensitivity suddenly jumps up and fluctuates greatly. It could also be an issue of mouse acceleration that occurs only in that view mode. By contrast there is no such problem with HM1.

This issue has been reported by various users on the Steam forum as well.
Post edited March 11, 2015 by leonardmccoy
Has already been patched: http://steamcommunity.com/app/274170/discussions/0/617329920705882457/
Wait for the GOG release.