It is possible that the coil is breaking down, but usually when a coil breaks down, it stops producing spark altogether and the engine just shuts off all at once. If the piston and exhaust is ok, then your most likely issue could be the carburetor that is not getting enough gas. It may need to be cleaned and rebuilt. But at this point, we are just guessing what could be wrong, but my best guess is the carburetor not getting enough gas in high throttle or the carburetor is getting too warm and having a VAPOR LOCK. Try using Premium gas and you may find out that that is a big help. All 2 cycle engine should use only 92 octane or higher fuel. let me know what you find out. Please don't forget to give me a good rating for my effort. This is the only way that this site pays me for my time. Feel free to continue contacting me if you need more information.