Small Engines (Lawn Mowers, etc.): Lawn Mower Stalling After Getting Hot, sears craftman, fuel pump

Question
I just bought a used Sears Craftman 19hp, 42" cut mower. It is in very good condition and looks to be only a few years old, which is what the man said when I bought it. I tried to Email Sears to try to find out exactly how old it is by the Model/Serial Number, but they wanted to refer me to a local Sears for service, which I don't want. The problem I am having is this: I can cut for about 30 minutes and then the engine starts to act like it is running out of gas. Revving up and then almost stopping and then back to revving up.  At some times it will actually cut off, but restarts when the key is turned. I went yesterday and got 2 new plugs and a new fuel filter, which the old ones did not look that bad, but haven't gotten to try it yet. The tank is full of gas and was almost full when I bought it. You think it could be water in the tank? I thought about that, but normally what I have found with water in the tank was that as the tank got almost empty, that's when it ran the worst. I'll inclosed the Model/Serial number just in case you have a way of telling how old it really is. Model # 917270781. Serial #021199D015141. I would very much appreciate your input on this and Thnx!

Answer
Try the plugs first.  Also, check the intake manifold bolts to see if they are loose.  I'd also check the fuel pump line to the engine block.  The ends may be swelling/expanding when the engine is warm preventing the fuel pump from working.

Let me know what you find.
Eric