When I play the game in the editor window and go down a hill in the landscape I made it freezes. What have I done wrong or is it just a glitch in the UDK?
Make sure to change the KillZ height (world settings).
Could you probably post your question into the UDK forum? https://forums.epicgames/forums/366-UDK 
Thanks that fixed it the hill was a bit lower than what I set the kiilz value to thanks.