Sounds to me like the octane rating in your gas has gone south. If your gas is really bad it should have a distinctive odor.
You're going to have to eliminate one thing at a time and Dave's suggestion is the best way to start. I am assuming it runs fine in neutral in the water and just starts acting up when under load. I once had a similar problem that turned out to be ignition related. If the fresh gas doesn't solve it look there. Good luck.
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