Prime Rib is one of my specialties. The key to a moist roast is always cook it frozen. This is a must. The 2nd most importanr thing is never ever poke holes in it with a fork. If you thaw a roast before cooking it you have juices and water on your counter and you end up with dry meat. Also when ypu poke holes in it again juices come out which will also cause your roast to be dry. My fovouite way to cook a prime rib is to barely cover the bottom of the roasting pan with water. Cook at 325 degrees and allow 1 hour for being frozen and then 35 minutes per pound for medium ( longer for well done, shorter for rare, only 5-10 minutes per pound either way.) Following this basic guideline you should be able to cut it with a fork. Also I like to coat the roast with BBQ sauce after it has been in 1 hr. I use kraft plain but any flavour will probably work.
Then 1 1/2 - 2hrs before it is done I add potatoes and let them cook in the juices and BBQ sauce drippings along with the roast. Stir the potatoes 2 or 3 times through the cooking process and make sure all sides have been in the juice. You can also add carrots the same way but they take about 1/2 hr longer than the potatoes and you have your complete meal in one pan and it makes an excellent tasty meal. I hope this helps.