I have a rider mower I purchased new in 1998. It has the Briggs and Stratton 14.5 HP I/C Quiet engine. This mower will run smoothly and mow my yard fine for about 40 minutes, then it starts to sputter and try to die out. I can play with the choke and get it running smoothly for another few minutes, then it sputters again. Play with the choke again and it runs fine for a few more minutes. Finally, it dies out and will not start again until it cools down for a couple hours. I have put every new part into this I can think of: starter, solenoid, magneto coil (properly gapped too), spark plug, gas cap, fuel lines, fuel filter, carburetor, air filter, you name it. The gas tank is semi transparent and I see no dirt or soot in the bottom. I have made sure there are no grass clippings stuck in the head cooling fins. What is causing this?