It could be a number of things. start first with the smaller things like changing your plugs, filter, and also run it on another 6 gallon tank and hose to rule out any hose or tank problems. you could check for fire but if a cylinder is missing you should notice it at all speeds. you could check your compression should be about 110 to 150 depending on the guage with no more than 15 pounds compression difference in the cylinders. with all these things checked next i would make sure carbs are clean. then on to something like a head gasket or even a power pack isue. hopefully the first few easy things solve your problem, if not my guess would be carbs. hope this helps, good luck.
Air leak in hose or fitting, dirty filter