Chuck, the comment about the gas cap could be a great tip.  Apparently dad lost the original gas cap along the way because now there is a nice shiny chrome one there.  Maybe it isn't vented.  I'm not sure that would totally match the symptoms the tractor is exhibiting, but it is easy enough to check - just lean up and briefly loosen the gas cap.  I do keep an eye on the fuel strainer bowl and occasionally drain it to let a little sediment out.  Thanks for the good tips!