Don, If mower gets used a lot, may just be belts slipping from wear. If all belts look good, there are few things that can cause your mower to loose power, bad fuel/filter/Air filter, carb. out of adjustment, failing coil running on one cylinder, valve adjustment required, engine worn/low compression, faulty plug, fuel pump starting to fail, etc. Two of the most common are fuel tank cap vent starting to plug(loosen cap to see if engine improves), the other common issue is a dirty carburetor, which will have to be removed cleaned/rebuilt . Good luck.
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