Reverse gear not working properly

My car it is facing reverse gear problem. In the morning the car goes in reverse gear but if it kept in. Cold about a hour reverse gear don't work. It needs to run in forward about 5 Km then the reverse works again and if reverse works for a time it always work until rest of more than 1 hour. So in cold it doesn't work but in garage morning it works and in hot or running also work. So please help me that what I can do.