In misc.rural DJS wrote:

One thing confusing to me is what part would fail that would stop the
mower when operating that also will not allow it to start.

Sounds like an interlock switch somewhere; if you get off the seat,
for instance, some tractors will kill the engine, and it also stops
you from starting it while not on it. It's consistant with the
problem you're describing, a frequent weak-spot, and logical, which
of course means it's not the problem on your tractor, but it's
something you should check first.

Dave Hinz