I saw something interesting a month or two ago, on a 100BaseTX interface between a firewall and Cisco 3600. A show interface on the cisco showed a high number or collision, but the connection was configured full duplex on both ends and the speeds matched. The cable was typed and certified Cat 5 UTP. The cables used were 6 ft - 15ft in length. There was not power lines or external noise near the cable run. When the interface is changed to half duplex the problem deceases drastically.
Greg @ Arlington