Has the mower been left outside? Most often, this is caused by a blob of water in the bottom of the tank (carb-on-the-tank engines), or in the float bowl ('Quantum' type engines).
FYI there is no good reason to warm up the engine for 10 mins. 1 minute is easily enough - just a waste of fuel to do otherwise, and certainly it should not be left idling. Cooling air flow is reduced by the square of the speed reduction, whereas heat production is directly proportional. IE 1/2 the engine speed, 1/4 the air flow. Conclusion - left to it's own devices. and idling mower engine will eventually overheat and blow the head gasket - or possibly cook the ignition coil, which IS a common failure, and may possibly be your problem.
phil_saunders(a)bigfoot.com
Phil Saunders
June 2012