Definitely check wiring and all switches, starting from the ignition switch, behind the panel.
Occasionally over the past year my John Deere Sabre would be difficult to start, and more recently every time I would fully engage the PTO lever for the mower deck, the engine would die.
$500 in parts and labor later (carb, plugs, fuel pump, fuel filter, battery) I discovered that the wiring harness/plug behind the key on/off switch was loose.
When bumped slightly by the PTO lever engaged, it would loosen and engine would not restart until the loose switch/plug settled back into place temporarily.
Another sign that it was loose wiring or a switch should have been that sometimes the engine would die while rattling around over a bump, even without the PTO lever engaged.