Had the same problem and replaced the ignition switch and away it went with a hiss and a roar .Theres a mile to much current going through the switch and the switch wont handle the load for long . i was intended to fit a couple of Relays to carry the load but sold the motor for a yamaha instead .
OMC is renowned for the ignition switchs giving problems . The solenoid for the starter was another thing also so changed to a standard automotive and the problem went away for ever . The contacts in the omc are to small and fizz in a very short time . I always carried a screw driver under the hood to short across the terminals when it wouldnt work . Got used many times if we were skiing and lots a stop start during the day .
I ended up taking the solenoid to bits to find out why they kept crapping out so quickly .