During playing on .61 exp I observed the following at least twice:

• Approach an over watch position of a remote town (optional with binoculars)
• Observe the area
• Watch for infected teleporting/moving really fast. The infected even appear to be much larger as if the models are pulled apart.
• within seconds everything settles and the infected walk normally

Today I even saw it from relative close range: a deer was stuck in a village and some infected did chase the deer but the movement was really choppy. Suddenly all infected who did initially chase the deer did aggro on me (I was still far away).