Are you losing power and hearing a gradual slowing for a couple of seconds before it dies, or does it go from running strong to dead in an instant? The former probably is a tuning problem, too lean on the top end. The latter is probably a plumbing problem, likely a damaged or slipped off clunk line inside your tank. Of course, it could also be an air leak at the carb o ring or backplate, so do check to be sure that bolts are tight and o rings are in good shape.

Also, what happens when you raise the nose of the plane with it running full throttle? You should hear the RPM increase a little and stay that way for as long as you care to hold the plane. That's an easy test for whether or not you have a plumbing problem, and also will tell you if you are tuned too lean.