Hello and welcome to the forum. Sometimes it is as simple as a loose connector. Maybe you could unplug the slide controller and plug back in. Being as it sometimes works it may not be to serious. Make sure you are on fast idle when moving slides in and out. This keeps voltage up going to the slide motor. Maybe the dealer can trouble shoot it, but you might think about a trip to the factory for warranty work, especially if you have a number of items to be addressed. Also, are you aware of DOAI, "Discovery Owners Association"? Great group of very knowledgeable Discovery owners. The forum is second to none for troubleshooting issues. If you want to know more about DOAI you can contact me off line or google Discovery Owners Association"
Good Luck
Jim Pinkerton 2006 Discovery