I had this same issue on the D5100 but I solved it with a class 10 card and about a one second period in-between each photo. This was sufficient time to get photos out of the cameras buffer and into the card.
Is your Card a real Sandisk? If you bought it online from a non reputable dealer (Ebay, Amazon, Ect) then it could be a fake.
Do you have the interval set right? If your doing a 30 second exposure with 10 seconds in between, then the interval should be set at 40 seconds. If you simply put 10 in for the interval, the computer will try to tell the sensor to take a photo every 10 seconds. Since you manually set the time to a 30 second exposure, you over ride the computer. What will then happen is the computer will still request a picture every 10 seconds even thou it's not possible. These requests will then queue up in the computer and eventually there are so many queues, the camera freezes and then resets the whole interval shooting session.
That's probably what was happening to you.
-Wahugg