Express Van turns off while driving

So I just bought a used 2001 Express 3500...Ive driven it a total of 150 miles and in those 150 miles it has shut off while I have been driving. Not at high speeds...both times I was going probably 15 mph. The first time it wouldnt start for about 2 mins...this last time it started right up. Any clue what could be the cause of something like this?