Why Fast Zombies and Slow Zombies?

Much of zombie behavior is going to be determined by how "fresh" their victim is, and the damage to the victim. A newly zombified human will still have major organs and neurons intact, allowing the zombie parasite to move and react as a normal human. However, as the bodily systems begin to break down, quick movement can become more difficult.

One of the first systems to begin to break down are those highly dependent on an intact neurological system, such as sight and balance. This Wikipedia article explains the fundamentals as well as any article:

Balance is the result of a number of body systems working together: the eyes (visual system), ears (vestibular system) and the body's sense of where it is in space (proprioception) ideally need to be intact. The vestibular system, the region of the inner ear where three semicircular canals converge, works with the visual system to keep objects in focus when the head is moving. This is called the vestibulo-ocular reflex (VOR). The balance system works with the visual and skeletal systems (the muscles and joints and their sensors) to maintain orientation or balance. Visual signals sent to the brain about the body's position in relation to its surroundings are processed by the brain and compared to information from the vestibular, visual and skeletal systems.

From my research, the vestibulo-ocular reflex actually begins to break down quickly. Although the exact progression varies, eyes seem to degenerate fairly quickly in a new zombie. One of the results of this is the well-known tendency of zombies to begin relying heavily on sound and smell over eye-sight. However, as it impacts their ability to balance the body, a zombie's movements also slow and become more herky-jerky as it corrects and over-corrects in an attempt to keep the body upright. A zombie that has started to decay will often fall over if it attempts to run for any distance, although some compensate by scooting along on all fours. There are even accounts of zombies scuttling around in a "crab-race" position. As its condition deteriorates further, the zombie may be forced to crawling or pulling itself along the ground.

As greater and greater decay creeps in, balanced movement becomes more difficult for the zombie. At this stage, the zombie will often become inactive waiting to suddenly lunge at prey. This not only conserves energy, but keeps the zombie from damaging itself or warning off prey due to its more poorly coordinated movements. Zombies in late stages are easily outrun, provided you can avoid the initial and sudden attack.