Either the clutch needs adjusting or the clutch/friction plates don't have any oil on them and they are sticking. My 250 did that when it sat for a long time. Try adjusting the clutch cable on the clutch cover first. -Mark
Finally a smart answer. The first two were just plain dumb. If it's not what Old_School mentioned, you should inspect the clutch for any worn parts?
But are you saying it just revs down and down and shuts off? Or it stalls with the clutch in? And is there a difference to when it's hot or cold?