dremel 300 stops when in high rps, why? ?

Help, I bought my dremel 300 last year and have not used it intensively, but a few days ago, while working with it at higher RPS it just shut off.. No smell, no burning. Took it apart, put it back together and it began working! just for a small while, once I put up the RPS it shut off?! Could it be because of the vibrations there is some contact that becomes loose once it begins vibrating more??