My sons 2 stroke 49cc dirt bike will run then stall out like it has no gas but start right up and do it Agin?

I had a Yamaha Blaster (200cc 2-stroke quad) and it did it all of the time. I never could figure it out. I would be going along in 3rd or 4th gear just cruising and then it just started losing power and died. I would usually be able to keep it alive by down shifting really fast and keeping the revs up. The only conclusion that I could come to was the fact that it might have gotten a little hot. Take it in to the shop and have them check it out. As a last resort, upgrade to the 4 stroke.