Yamaha 90 Reving Problem

[Brad the pilot]

My 1990 Yamaha 90 outboard runs great until recently. Put it in neutral after an uneventful 45 minute cruise at 4000 RPM and it all of a sudden it reved up to 5000 RPM with the cockpit control in neutral. Shut it off with the key & restarted and it reved again so I quickly turned off with the key. I was away from home so started it and put into gear as soon as it started so it would not rev before shifting and risk damage to the gears... Any thoughts?