20hp walker mower shut down?

[Dan D's landscaping]

Hi i have a walker with a 20hp kohler motor that is shutting down when gets hot was wondering if anybody else has had this problem. start up run's great then shuts down after after 5 min and other times in few hrs. no overheating light goes on just as if you shut key off? also noticed that that the breaker gets hot when this happens and clicks on and off? if you jump a screw driver across it when it is doing this get sparks on it but only when it goes into this shut off problem when it is normal running you do not get this. i have replaced the key ignition and that did nothing. any help would be great. thanks