every now and again while sprinting or moving up hill the player character will slow down or stutter in motion preventing sprinting and often slowing the player down to moving almost like they are maximum fatigue (IE the one handed slow walk just without the animation).
This can be unreliably treated via having the player switch active weapons.
Jog either up a very steep hill or across a wide flat plane without switching weapons until fatigued then walk along uneven terrain while trying to jog.
SHOULD NOT BE CONFUSED FOR NORMAL UP HILL / FATIGUE SLOW DOWN.
This problem and its work around have been reported by other players as well at-least to myself personally.
Unable to reproduce in the latest dev build.
Since this ticket is very old and has not recieved any attention for a long time, I'll go ahead and close it. If the issue is still reproducable for you, please create a new ticket.