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It sounds like you have made all the right steps so far, the only other thing is to make sure that the burner is properly adjusted (not overheating). Check the t&p valve if you can safely do so, to make sure it is operable and that you don’t have steam in the top of your tank.
Also if you replaced the anode, go ahead and drain some water off the drain tap to make sure you have no aluminum residue from the original anode.
I will assume that you have consistent water pressure in the 60-65 psi range, and that all the water lines are strapped.
AO Smith is a good company, and they stand behind their products, I would contact them, like I said you have done all the things that would be in line with this sort of problem. AO may want to send out a factory tech. to look things over.