I have the same problem. I have to get my friend to stop and wait for me when I am in full sprint despite the fact he is crouch running. It is definitely not cause by the carrying of items. It is evident when in 1st or 3rd person mode. When trying to sprint with zombies chasing me they tend to run past me and stop and hit me while I try to go past them again. I don't think it is related to fps or ping. My friend describes my movement as all action without really going anywhere. Perhaps related is the appearance of tiny white dots visible across the grass textures. they don't appear on buildings or in towns. A fix would be great