You might find that is your problem. If you look at the chain cogs and see that the teeth are pointed at the end you need to buy new cogs and a new chain. If the tips still heve straight edges, your chain could be very loose. Either way you need to sort out the problem as a broken chain at high speed is no fun. To adjust the chain, loosen the rear wheel and turn the adjusters to tighten the chain until you have about 3/4 inch freeplay. Make sure that the indicators on the side are against the same marks to ensure that the wheel is straight.

To replace the cogs and chain remove the back wheel and tap down the tabs that are used to lock the bolts in place and undo the bolts. To remove the front cog to will need to take off the cover. If your chain does not have a master link, the easiest way to remove it is to cut it off. Unless you knoe how many teeth your gos have, take them and the chain into a bike shop to get the right parts.